Not at all true or well advised. These are the arguments of someone who's trying to justify what they already want to do. If you found a car that was unattended, even if you could reasonably determine that it ended up wherever it did against the will of the owner (e.g. lost, stolen or otherwise inadvertently abandoned), would you also think you could legally just assume ownership of it? Under the law, why would that be different? The fact is that finding unattended property does not mean it's legally up for grabs, and that is a fact. We should all know that Apple famously pursued this with the lost iPhone 4 prototype.
Seriously, what would you want someone to do if it was yours and you lost it?