When Ryan Wolfe uses the Ground Penetrating Radar to locate the gun under the concrete pad, he says he has a reflection at 18 inches deep. But the concrete pad was only about 3 inches thick and the gun was right underneath it, so the reflection should have only been about 4 inches deep.
The reenactment shows debris covering the entire vault floor and the larger chunks blown several feet into the middle of the vault floor. When the CSIs are in the vault, the large debris is piled next to the hole in the wall and there is only a minor amount of dust and small debris on the floor leaving it mostly clean.
Towards the end of the episode, Detective Tripp searches a "Gun Registration" database to find the owner of the 9mm discovered under the concrete. The state of Florida does not register guns, nor do they have a database of gun serial numbers.
Even though Florida may not register guns, the NCIC (National Crime Information Center) holds records of weapons in association with crimes or that have been stolen. They could have run a serial number through it, not just a Florida database.