Every entity on the ship - including your Deadnauts - can be detected using a number of methods.
To see how visible you are, check your stealth profile on the Crew console in the DEF mode. Similarly, you can check the stealth profile of any threats you discover via the Library console - provided you've discovered it.
Stealth and detection go hand-in-hand. Once you know an enemy's stealth profile, you can equip your Deadnauts with sensors that allow you to detect it faster.
Similarly, you can also invest in cloaks. These are similar to sensors, but they have the opposite effect: a Deadnaut's stealth profile will be diminished, making them harder to spot.
IMPORTANT: The higher the stealth bar, the more detectable the entity is. For example, if you encounter a threat with a low thermal bar, then it will be harder to detect. You will be able to detect it better if you have a high thermal sensor.
Some sensors can be equipped in a Deadnaut's primary slot. You can then select your Deadnaut and order a special action.
This will 'throw' the sensor to another location, revealing anything inside - assuming the sensor is able to detect it.
Sensors equipped in the passive slot will periodically emit a sensor pulse that radiates from the Deadnaut.
A cloak reduces a Deadnaut's stealth profile, making them harder to detect by enemies. This has the obvious benefit of making your Deadnauts safer - an unaware enemy won't attack. But attacking an unaware enemy can also increase the chances that your Deadnaut will perform a special damage effect (eg, slow, stun, confusion).
Using a projector, a Deadnaut can send an image of themselves to a remote location. The location must already have been revealed to the Deadnaut (ie, they cannot project into an empty void).
The projection will then try to move through the nearest open door. If any threats are in the room, they will follow and attack it.
The projection will deteriorate over time, and it will deteriorate faster if it is being attacked. Unlike scanners, projectors do not reveal furniture, sentinels or enemies.
The below image shows a projection - the yellow icon - luring threats away from the Deadnaut.