[CORed]

From the technical standpoint, it certainly would be possible for them to install come kind of spyware that will report back to them periodically over the internet. It is also conceivable that they could put in some kind of hardware device to do so. However, I think that most likely they did neither. My guess is that they simply copied your hard drive, so that they could examine it for anything incriminating at their leisure.

If you're really worried about it, you could reformat. That would probably disable any spying software, but probably wouldn't affect a hardware "bug".