Dumbest question ever. It's theft when it's not yours.
Aaaand once again a friendly reminder that MR has an international following but not all countries and cultures use the same legal terms as California State Law does. They might refer to finders-keepers situations like these as something that would perhaps translate to "misappropriation" or "unauthorized acquisition", while reserving "theft" for situations where shady characters with mask, swag-bag, striped T-shirt and crowbar are the perpetrators.
It does sound like a reasonable thing to do since "theft" and "finders-keepers" crimes are rarely committed by the same category of people. A professional thief isn't going to sit around in bars hoping that someone will leave stuff behind (it could take years), and a finders-keepers-minded person isn't going to actively snatch stuff from people since they don't live a criminal lifestyle, they typically live on the right side of the law until one day when they're 21, or 45, or 80, and they suddenly spot some thing just lying around with no one in sight.
Imagine for the sake of argument that you had read an interesting news story from the alien planet Zorx about a guy who died from a water balloon having been dropped on his head, and since the Zorxians have a 20-inch fontanelle, that balloon just crashed in and short circuited his brain.
So, you go on their Zorx Rumors message board where people are discussing the case. You're not a Zorxian so you don't quite know how to join the conversation, but what the hell, you have to start somewhere, so you write:
"Hi, so was this manslaughter or negligent homicide, or maybe it'll be written off as an accident, what do you guys think?"
Immediately a lot of p*ssed off Zorxians who are frothing at the mouth from indignation pounce on you and go:
"Dumbest question ever. It"s first degree murder when someone dies."
"Really?
Murder?", you respond. "Sounds a bit over the top, don't you think? No chance for, say, reckless endangerment? I mean there was no premeditation or intent or anything of the sort, right?"
"Oh man *eyeroll*, the ethics and morals in this world are going straight to hell", says another Zorxian. "Now they're making up excuses for this cold blooded murderer".
Eventually, it dawns on you that the Zorxians simply don't HAVE any other death-related legal terms than first degree murder. They've simply taken manslaughter, negligent homicide, assisted suicide, second degree murder and-what-have-you and rolled them all into one. Does that make you or them stupid?