It's pretty common for lube shops to put a little extra in, what kind of overfull are we talking about? If it's the injection pump, it will be a bit involved. All the hard lines have to come off, as well as the steering pump and vacuum pump. Your oil will smell a little funny just from running and having soot from blowby, but if the level CLIMBS it is your vp leaking. The injectors can only leak a tiny little bit, and it would be more of an issue with starting. Most likely a leaking injector would leak into one cylinder on shutdown. This is much more common with common rail engines, but not unheard of in mechanical pump systems. (The vp is still mechanical, just computer controlled)
In a common rail system, the offending injector will empty the entire rail's pressure, but in a mechanical system, it can only leak the pressure from the one pressure line, and only if the engine stops with pressure in that one line.
I would make sure it's actually climbing before doing anything, but if it is, I would guess it's probably the front seal on the vp.