Flying 2'000ft below an Airbus A330 (FL390) on the same routing I noticed a contrail-like vapor trail forming behind the fuselage. It was only very faint, so not visible from the earth's surface, and it only formed for a few seconds, then vanished, only to reappear roughly a minute later or so.
The reason for this mini fuselage-trail remains unclear to me, yet I have 2 theories:
1) Water released from the fuselage. The origin of the water? Don't think that water drained through galley sink would form a visible trail. So maybe condensation water from the Air Conditioning unit? But why would it only appear on intervalls and not constantly? And it would probably not be that much water released either...
2) Air relesased from fuselage. Maybe also through A/C unit - warm (cabin temperature) air would be humid and at high pressure compared to outside air, so would rapidly expand and maybe form fuselage-trail. Altough, as far as I am aware, this release of air happens constantly so no explanation for the intervall phenomenon.
Didn't mean to mislead you with my theories - maybe the reason is something else?
...who has the correct answer?
Thanks for your comments!