As you scroll through the Marketplace, you will see that Every BattleMech and some Vehicles have 4 Numbers </br>attached to them. Sensor Power, Sensor Range, Stealth and ECM. 
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Lets look at a Locust.... </br>
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Range: 2 Base sensor range (BSR) is 4 hexes per level. It is possible to detect units at multiples of BSR </br>
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Power: 5 High power allows more likely detection at higher multiples of BSR, and counteracts stealth and ECM </br>
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Stealth: 2 Also includes from basic camo to advanced color-changing paint and light bending technologies </br>
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ECM: 0 Electronics countermeasures jam enemy sensors. The highest ECM rating per side is used in any battle </br>
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Now, to explain how Sensors Work... </br>
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A base roll of 2D6 is used to determine how far out you can "detect" units.... However, this 2D6 is modified by the following: </br>
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base adjustment is Power - Stealth - best ECM - 2 </br>
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So, it takes your Power Rating... Takes away the Enemys Stealth... Takes away the BEST Enemy ECM Rating on the BattleField.</br>
Then Takes away 2.... </br>
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This gives you your Base adjustment... Now, this BASE adjustment is further modified by the type of target, and what it does,</br>
 in the following manner: </br>
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infantry are -4 to detect with sensors. Radar Waves tend to have little effect other than a Carcinogen to Infantry. </br>
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vehicles are </br>
Light: -2, - Lets face it, a 2m Jeep or a 15m Demolisher? </br>
Medium: 0, </br>
Heavy: +2, </br>
Assault: +4 - What did I say about the Demolisher? </br>
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mechs are Light: -1, Easier to hide behind trees... </br>
Medium: +1, - Getting Larger... </br>
Heavy: +3, - Lets face it, they aren't subtle at this size... </br>
Assault: +5, - Can you say "Jurrassic Park" ? </br>
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Target is Stationary -2 </br>
- Be vewy, vewy qwiet... </br>
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Enemy movement .. up to +5 (+1 per 2 hexes above 1) </br>
- The Faster it goes, the more like it shows up against a "Stationary" Background </br>
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Jumping = additional +3 to be detected.. </br>
Pretty Self Explanatory. </br>
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Heat ... +1 per 5 points </br>
- Neveron Sensors include a full suite of Thermographic scanners... A Flashman who Alphas isn't going to appear like an Ice-Cream anymore. </br>
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Fired last turn = +4 to be detected </br>
- "Now, let me see... I was hit in the back, so....." </br>
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Previously spotted.. +2 </br>
- Lookie, Lookie... Lets play Follow the Leader... </br>
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inside buildings remember are -2 to -8 depending on CF </br>
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each sensor crit = -5 </br>
- Yep, you're blinded alright. </br>
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Now we've got the total adjustment to the roll. Neveron then takes a Roll of 2D6, then applies your Total Adjustment,</br>
wether positive or Negative - at the end, you have your "Sensor Roll": </br>
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If you roll is 2 or more, then you will only detect units at 0.25 times your base range... </br>
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If you roll is 4 or more, then you will only detect units at 0.5 times your base range... </br>
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If you roll is 6 or more, then you will only detect units at 1 times your base range... </br>
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If you roll is 8 or more, then you will only detect units at 2 times your base range... </br>
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If you roll is 10 or more, then you will only detect units at 3 times your base range... </br>
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If you roll is 12 or more, then you will only detect units at 4 times your base range... </br>
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If you roll is 14 or more, then you will only detect units at 5 times your base range... </br>
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If you roll is 16 or more, then you will only detect units at 6 times your base range... </br>
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If you roll is 18 or more, then you will only detect units at 7 times your base range... </br>
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If you roll is 20 or more, then you will only detect units at 8 times your base range... </br>
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Remember, that each 1 Point of "Base Range" is equal to 4 Hexes on a Map... </br>
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So, lets say two locusts are trying to detect each other. Locust A stood still, while Locust B ran at its maximum of 12 Hexes.</br>
They stand 10 Hexes apart, but with Hills in the way, Obscuring Line of Sight... </br>
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Locust A: Power is 5, the Enemy Locusts Stealth is 2... Since there is no others on the Map, the ECM is 0... </br>
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base adjustment is 5 - 2 - 0 - 2 </br>
So the Base adjustment for Locust A is 1. </br>
1 - 1 (Enemy is a Light Mech) +5 (Which is moving damn fast) </br>
So Locust A's Sensor Adjustment is +5... </br>
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On the 2D6 Roll, Locust A scored a 4 - so its total Sensor Roll is 9. Which means it will detect the unit at 2x its base Range.</br>
As its base Range is 2, it can detect at a Range of 4, which is 16 Hexes.... Since they are only 10 Hexes Apart, Locust A sees Locust B...</br> 
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For Locust B.... </br>
base adjustment is 5 - 2 - 0 - 2 </br>
So the Base adjustment for Locust B is 1. </br>
1 - 1 (Enemy is a Light Mech) -2 (Which is stationary) </br>
So Locust B's Sensor Adjustment is -2... </br>

On the 2D6 Roll, Locust B scored an 8 - so its total Sensor Roll is 6. Which means it will detect the unit at 1x its base Range.
</br>As its base Range is 2, it can detect at a Range of 2, which is 8 Hexes.... Since they are 10 Hexes Apart, Locust B fails to see Locust A this turn. </br>
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The above applies if you do not have Line of Sight... If however, Line of Sight CAN be Drawn to your Enemy, it works slightly differently. </br>
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If you have LINE OF SIGHT: </br>
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Partial cover = +3 </br>
- Always harder to see a Half-Hidden 'Mech </br>
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Woods = +2 per point </br>
- Can't see the forest for the Trees, eh? </br>
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No heat mods, or Sensor crit mods, or ECM </br>
- Critical Hit! Eyeball Mk. 1 - Owie. </br>
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Height = +3 levels for jumping, 1/2 height for prone mechs </br>
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Previously Spotted = +4 not +2 </br>
- If you're already relying on your eyes, so much the better... </br>
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With line of Sight, you use the same formulas as above, except disregarding the ECM value, Sensor Crits, Heat, and add in the</br>
Partial Cover and Woods Modifiers... You use the same range target numbers as above, but your Base Sensor Range is considered to be 20. </br>
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You can always see enemies which are in your hex, or in an adjacent Hex... Additionally, Line of Sight, as well as Sensors, </br>
are both 360-degree items... </br>
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I hope this has shed some light on the "Sensor" Mysteries... </br>