Ok this will be the first mission where the sensor skill will really come into play with regards to enemy units.
Here is how it will work.
If you have a Mech in Line Of Sight and within 54" at the end of the a given active enemy units movement phase, the enemy unit miniature will be placed on the board. (I came up with the 54" from the longest range weapon you have available. The clan Ultra AC2 on the Koshi)
Yes this may create some times where you move around a corner first to reveal what is behind there, then the enemy moves back out of LOS during it's move, I will place a wooden block to mark the unit. I don't expect this to happen in this battle, but will be much more likely if we get into a city fight.
I will use common sense for this as well. If during movement an enemy unit zooms from one piece of cover to another through an open field that is within LOS of 3 or 4 of you guys, then yes I'll probably place a wooden block marker.
Your mech is not like a WWII tank you do have some good computers and sensory equipment on it. If an enemy unit is not in LOS, but within 12" at the end of the movement phase, I will place an enemy unit marker (A wooden block). That way you will know where they are. I will use the wooden block for movement the next turn.
But what about the Sensor Skill and Active Probes?
Now for sensors. Anyone with the sensor basic skill or an active probe on their mech has the range increased from 12" to 24". Pillsbury has an Active Probe on the Koshi so his non-LOS spotting will be 24".
The Sensor Skill stacks with the Active Probe. So Poe has the Sensor Skill and the Raven has an Active Probe He will have a Non-LOS spotting of 36". In addition Poe will be able to tell if the unit is a Vehicle, Infantry or a Battlemech. (At higher levels he will be able to tell the weight class of the mech or vehicle, or even the exact unit at the top sensor level)
The idea behind these rules is to add suspense and a level of tactics to the game. They don't come into play often, but when they do you will see having the sensor skill is quite handy.