I’d secure it myself, then bring it into the nearest police station. First, however, I’d photograph the hell out of where I found it, and even try to find a witness or two who’d swear up and down that my story was right.
The big downside to the legal doctrine of finders-keepers in this case is what happens if you eventually do have to use it to protect yourself. The inevitable investigation would illustrate the principle of “compound interest” in a way that even the most math-phobic would immediately and completely understand.
I would leave it in place, keep others away, and call LEO if at all possible to do it this way; it may not simply be a misplaced firearm but could be evidence in a crime.
It is hard to tell whether it is simply misplaced or has been ditched by someone who had a reason to ditch it.
I would not touch it and I would call the police to take custody of it.
I'd feel pretty bad if I had discovered evidence that could put a bad guy away for a long time, but somehow messed things up through my actions or inaction and gave him a free pass.