You argue that aliens would set off a heat signature, but the phenomenon we have observed move without any friction with the atmosphere, no excessive heat signature (They have been seen on thermal, but it runs somewhat cool) and no sonic booms.
If someone has that kind of technology, or if aliens exist and can do interstellar travel, there is no way to be sure their means of travel or power production really set off a noticable amount of heat.
And I'm somewhat skeptical of the claims of aliens.