"so just wondering would they of put a device in my PC ?"
Have you discussed this with the solicitor, because I cannot see any reason for the police to take one computer away, and leave (what you suggest) a possible tracking device on the one they left behind.
You could possibly do a few checks yourself, by running any 'anti' programmes you have installed on the computer. By 'anti', i mean general security or protection like Avast, ADW, SuperAntiSpyware or similar. This simple system of checks might help with your concerns!.
Regarding your son's activities, then again your solicitor should have any answers to the reasons why. As he been downloading or distributing things that perhaps he should not have done?.