UFOs cruising over Phalanx installations ?

[anonymissimus]

Sometimes UFOs are flying back and forth over a base (or apparently phalanx installation in general) and aren't doing something in particular. It looks as if they are waiting for a phalanx craft to pass by which they can attack. In the save I will now lure the UFO towards my ground rockets to shoot it down. It's a harvester over a radar tower (!)...what is this supposed to be ? Bombing failed because of a bug ? I read that that can happen but it never did for me.
EDIT
It was eager trying to shoot down my Saracen and is now wrecked.

[ShipIt]

Aliens attack installations, too. Sometimes it takes them some days to finish the work. Cannot find the ticket right away, but iirc this is a known problem.