The GC3 tamper switch is up at the top right on the inside, with a small rubber cover. Make sure the cover has not been damaged, and make sure that no cables, etc. are blocking it from being closed. If the top right latch is not hooked on properly when re-hanging the panel, you may get intermittent tampers. A protrusion on the back-plate depresses the tamper switch when closed. Make sure the back-plate is level against the wall. If it is uneven this may occur as well.
I see a lot of tamper reports after this posting. Were you working with the panel at that time?