[tower]

Yes, they are held in place by a hoop spring which presses outward on 8 plastic teeth or fingers. the friction of these teeth against the inside of the wheel center contour is the only thing holding the center caps in, so yes, they DO have a tendency to fall out from time to time. The advice about pulling them for off-roading is good. I have not done it, but have lost caps spontaneously on the road. Checking each one to ascertain that it is properly seated is also a good idea. I have thought (but not yet tried) gluing a rubber shim (cut from an old bike tire) to the outside aspect of one or two teeth to maximize friction.

You might also inspect the caps and make certain that the hoop spring is seated properly between the large and small teeth.