Gas Tank issue...

The only holes in the tank are the filler neck hole and sending unit hole. From what you described I'm with you on thinking that the grommet is either not there or poorly installed or not replaced the old one. There could have been an issue with when installing the tank the grommet got hung up on the filler neck and got pushed into the tank. If that happened the po may not of known it happened.

Good luck and hope it is just a minor fix with the grommet.