Hi!
No, it's certain that it's on the uplink. The weaker stations are having trouble getting in, and the signals disappear when the satellite pass is over. The strength of the signal is closely related to movement of the satellite, and my S-meter doesn't change vs the satellite when the signals are there.
This is the same thing here. In fact, on a couple of occasions, I would call a ham friend who had also worked the same satellite passes to confirm what I heard was something he also heard. As you mentioned, I would hear the other chatter and I could tell it had to be coming from the satellite downlink and not from local simplex activity. The foreign chatter will usually go away as the satellites move northward, but the other stuff has sometimes stayed on AO-27 for most of its 7-minute passes. The QRM would follow the Doppler shift, not stay on one frequency regardless of the satellite's movement.
I remember a couple of weeks back clearly hearing a CW ID on an AO-27 pass. I think I might have that pass recorded at home, and will try to locate that recording. I seem to remember it was a US 9-call.
We in North America don't have it as bad as our Australian and European friends who deal with all sorts of intruders. If the voices are hams mistakenly using a satellite uplink as a local simplex frequency or as part of some repeater/remote-base system, some polite persuasion should be employed before involving the authorities.
Back to work... 73!
Patrick WD9EWK/VA7EWK http://www.wd9ewk.net/