I tried sort of rolling the case from side to side, but heard nothing.
I have a torch with a center extendable antenna like magnet picker-upper ready, but no joy so far.
I have a Phanteks Primo (big and heavy) case, and I think the two little screws dropped down through one/two of the bottom 140mm fans, and now that the PC is fully built, it will be a bit of a bugger to dismantle those two fans to see if I can find the little screws.

The PSU is mounted on its side with the fan facing the filtered outlet grill, and is on the bottom on the other side of the chassis enclosed in a metal shroud, so there is no way the screws can get any where near the PSU.
So I figured to try and source some replacement screws, and it is not going well....

Mean time, I have two Samsung 950 M.2 Nvme 256GB cards feeling all left out and sad...and lost their screws when I removed them when I was trying to get Windows 7 installed (yes I run that because a couple of sites I use will not work with Chrome or Edge), but I also run Windows 10 on another drive.
It all got a bit complicated, and caused much use of Paracetamol trying to get every thing up and running.

Looks like I am stuffed then....