And to those asking why nebulae - it's quite logical if you think about it
Thinking mode engaged - Random ideas time.
IDEA 1:
Barnacles are "eggs" of aliens (kind of makes sense since they are strong etc). For some reason the aliens lay their eggs in nebulae, maybe to be warmed by the hot blue stars, maybe because there are "nutrients" around (perhaps the aliens are organic ships). UAs might be some kind of perimeter detection system.
Look for a nest site. Would make for a very interesting time when the aliens find us collecting the eggs.
IDEA 2 (even more wacky):
Aliens live in stars. Stars explode, cause nebulae, also spew alien skelingtons onto nearby planets. UAs are some kind of beacon from the aliens - knows morse code because we keep putting nav beacons around stars.