sounds like my son’s Millennium. We wEre at the range and after about a magazine and a half, the little panel covering the rear-end of the striker mechanism just popped off. didn’t seem to affect the operation of the gun but after putting the cover back on he traded it for an XD.
It’s been so long since I had it, but if it’s like a Glock, etc, that cover is how you replace or take out the firing pin, etc. I know on a 1911, if that piece comes out, the FP comes flying out.
I really really like my XD!