The idea of getting busted over the gun being stolen is absurd. The OP is at least twice removed from that. As I said before tho, the problem I would have is that the gun was not really the gifter's to give. I've lent guns to friends that have kept them for years(deer rifles, shotguns). While those guns are not upper tier or worth tons of monies, I would certainly be upset if they gave them away or sold them. I might even consider them stolen if done so. I don't know the situation of the gifter or the person that left the gun in the gifter's possession. I do know that I myself, would not gift something that belonged to someone else, without that others person's permission. Ain't much of a gift if it's not really yours in the first place. There's a lotta greed when it comes to firearms, both with possession and obtaining them. Sometimes the idea of obtaining them is stronger than what's right, legally or morally. What good is a bill of sale from someone who does not own the property in the first place?
Few years back I worked with a guy who used to drive truck cross country. One night at a truck-stop in Montana, he found a Ruger 9mm laying on the ground in the parking lot. When CWC became legal here in Wisconsin back in 2011, he asked me about what he had to do to get a license. Not being much of a gun guy, he also asked about the risk of using that "found" gun as his EDC. The idea that the gun was either stolen or used in a crime and then "thrown away" was his biggest concern. I asked the local Police Captain(friend of mine) about running a stolen gun check on the serial number and he told me they don't/won't do that for the general public. He also told me that the risk of being charged/associated with a crime from the found gun, other than what he did personally did, was nil. He also said if the person was really concerned it could be taken to the police station as a found gun, where the numbers would be run and if the gun was not stolen, and not claimed after a period of time, the gun would then be his legally.
Sorry, the gifter might be a good guy, but gifting a gun that still belongs to someone else is not the same as a widow gifting a gun that belonged to her now dead husband.
JMTCs.