RE: So what happens...
When you are connected to the "picocell" or "femtocell" the phone will have a different identifier showing on the Screen (called an Alpha Tag).
So if you were on the picocell and the operator was O2, you may see "O2 - pico" perhaps. If not then you are camping on a Macro cell.
Also any traffic flowing between the picocell and the network will over an encrypted communications channel. I would not expect anything different...Hacker's would have a field day otherwise.