I had this idea for a fictional scenario in which a forgotten satellite, like a communications satellite that was decommissioned because it was replaced by a newer model, was hacked into and then smashed into another satellite.
And then I realized I have no idea what happens to high-altitude satellites when they are no longer used. Are they completely shut down so that communication with them is no longer possible? I assume communications are secure, passwords and cryptography and stuff, but are they monitored so that hacking attempts would be detected? If they are decommissioned with some fuel left, are they still parked up there or will they be deorbited? Are any satellites, in fact, ever decommissioned before they run out of fuel? Nothing I've read has seemed to quite cover any of that.