I'll agree with the spacecraft venting suggestion: back in 2011, I saw shuttle Atlantis on it's last-ever orbit pass over Adelaide, chasing the ISS from which it had just undocked and completed a mutual-inspection 'fly-around'.
Exhaust from the shuttle's RCS maneuvering jets surrounded the ISS (you can see this is they pass behind a convenient 'sucker hole').
https://www.youtube.com/watch?v=Qeb2ix6HdpA
30 minutes later, the shuttle had de-orbited and landed in the US for the last time.
Gob-smacked would put it mildly!
Dean