if we are being watched by aliens, can't we just emit photons in a state of superposition and see if someone observes the photons? this can work because if someone detects one of our emitted photons, then we'd notice because the other photon that it's entangled with will collapse from its state of superposition. this works no matter how stealthy the spy is.

just create pair of entangled photons, one photon polarized vertically and the other horizontally, then broadcast one of them out into some specific direction of seemingly-empty space, and trap the other photon. after some time like a day, just release the trapped photon and see if it's still in a state of superposition or not.

if the photon is still in a state of superposition, then the other photon that's flying away into space hasn't been interacted with yet by anything. then you can just retrap the other photon to keep it in the state of superposition until the next day. if retrapping it is too hard, just emit another entangled pair of photons into that same direction and then wait two days next time before releasing the trapped photon.

now if the trapped photon is released and behaves like it's collapsed out of superposition, then you know that something in space interacted with the entangled broadcasted photon. it could just be a piece of dust, or rock, or aliens.

ideally, you would have a dense net of photons being broadcast outwards so you'd check the photons that are near that vicinity by seeing if their entangled partner has also collapsed. If the net is dense enough, you'd be able to tell how big that object is. If you broadcast these photons out frequently, you'd be able to tell more precisely when the photon actually hit the object.

this system can be used for more mundane things as well like detecting dark asteroids that we can't see, or even enemy stealth satellites.