I don't know what the lost and found laws are in FL, but here in WA the proper procedure would be to inform the owner of the theater, take the weapon to the police station and turn it in (with a claim tag). Wait 60 days. If the police cannot identify the lost property's owner, and determine it has not been used in a crime, is stolen, etc.. it then become the finder's legal property.
In this case, I am sure LE would determine who actually owned this firearm and would return it to it's rightful owner...
Example: (not quite a Glock, but worth more) Our youngest daughter found a diamond engagement ring worth about $4000 in the Mt Baker National Forest. From the style it was probably from the 30's or the 40's. She took the ring to the forest service headquarters in Sedro Woolley, and turned it in. 60 days later they called her up to come claim HER property. No-one had reported loosing such a ring, or were there any lost ring reports on file. That happened 30 years ago. She still has that ring.
If you play the game by the rules, it is sometimes possible to legally get quite a windfall.