[Klemm]

Are you at, or near an airfield, perchance?

The behaviour started after i let the AI take off and then continued for some minutes (i ran 8x time acceleration, so only for about half a minute in real time). Sorry, i don't know if i had reached the first waypoint after i turned off the AI and flew the mission for myself.
In any case i was at least 10 to 20 km away from the airfield i started from and the behaviour continued.
Oh, and i Just noticed that this AI behaviour stops when it gets over 500 meters or so (gotta check more closely).