Am I right that sensor does not help to discover an impersonation or holoecho marker? But I can detect troops in Hidden Deployment? I thought troops in Hidden Deployment are supposed to be not on the table (yet)?!?
By declaring Sensor, the user may make a Normal WIP+6 Roll (without Range or Camouflage and Hiding Modifiers) to simultaneously Discover all enemy troopers, Weapons or pieces of Equipment in the Hidden Deployment, Camouflaged or TO Camouflaged states inside the user's Sensor Area. So yes, it will reveal troopers from Hidden Deployment state, and no, it has no effect at all on IMP markers or Holoechos.
Hidden deployment troopers are not on the table as models or markers, but they still have a very specific position on the table, determined during Deployment. The distinction of not being models or markers not only makes these troopers hidden in a physical sense from the opposing player, but more importantly they are not affected by things that affect models and markers, such as template weapons. Sensor, however, will discover troopers in Hidden Deployment whose pre-determined location on the table is within the sensor area. In contrast, Airbone Deployment troops are very much 'off the table' in a real sense. The troopers have no location on the table until their are placed by spending an Order and following the rules for whatever flavor of Airborne Deployment is being used.