Randy Duck: Suggest that you look at the C172 Flight Manual, Section 7, Figure 7-7, Electrical System, and you will see that it is standard practice (at least for circ 1978 Cessnas) for the hour meter to be wired directly to the battery via a fuse and an oil pressure switch. Can't speak for the later C172's but as far as I know it is a standard Cessna setup.
"something we've all done... " Not too sure about that.
Why would someone so "intelligent" do something so dumb. The chances of getting away with it are slim to start with, the tacho times would not tally neither would the fuel uplifts. May be he is intelligent but not too smart.
Why did he own up? (Dob himself in?) Maybe he had two choices, we have caught you out, own up and apologise or we tell the police?