It might just aswell be glued, those caps are notorious for getting loose on it's own. That's why if you have 1d(s)3 and still have the cap in place, it's either glued or lost. My mkIV has a different rubbertype now.
The reason for glueing is obvious, very few use the terminal. but almost all of the owners take their camera out into moist conditions and heavy rain.
Try using a small flathead screwdriver in one of the bumps (do not know the english word for that
) in the cap and push slow and hard.