So, the Sensor is something that was always in the back of my head and I'm sure I would have assigned it eventually.
Gameplay theory wise, the rough idea is that aliens have great mobility that allows them to maintain map control by patrole and reconnaissance and potentially spamming lots of small forward bases for short pathes back to battle in case they get defeated. Humans are much slower and most of the time the effort of maintaining more than a single (strong) forward base won't be worthwhile to them. (Ishq just pointed out that we shouldn't force them to do so either.) In order to balance map control given the human's general disadvantage in map coverage, there are mainly two things we can do: Allow humans to disrupt alien movement with area denial weapons and give them means of map awareness that doesn't require patrole. The sensor would do the latter.
Gameplay praxis wise, we didn't yet talk about a specific implementation but I'm certain that we'll find one, given the possible attributes (small/big range, detailed/rough intel, an actual buildable or a deployed item, per-player or per-team). Currently I image it to have a very small range, covering approximately the width of a standard corridor. When an alien passes, it would issue some kind of warning. Ideally we will at some point be able to display blips on the minimap instead (and pointers at its edge, given the target is outside the current excerpt). In thise case we can increase the range and make it like a watchtower in a standard RTS (except it doesn't shoot).
Getting to the appearance, the Sensor should be a rather small buildable that you can hide in corners. It has a spherical area of detection around its center, so it should reasonably look like it senses in all directions at once. The feet shouldn't extend too far (as opposed to many real world sensors) so that the bounding box can be a tall cuboid. Speaking of real world sensors, it seems they are called Unattended Ground Sensors in military jargon. This is potentially a good source of inspiration. Another thing that our sensor should have and that you wouldn't find on a realistic one is a small control lamp, visible from any angle, that will glow up whenever it detects something. This will tell alien players when they've been spotted (in case they see the sensor at all) and tell close humans that they should have a look at their minimap (or message list) to learn about the enemy that is very close to them.