If you find $10k in cash in a bag
If you find a $10k car parked on the street
If you find the latest iphone sitting on a table
I get what you're getting at but I'm saying that there are plenty of details that matter regardless of ease of proof that the object isn't yours or whether or not it's foreseeable have been left there under "reasonable" circumstances
Cars and phones are trivially easy to track back to their owner, with cash possession is basically ownership. Cars get parked places, phones get left places, cash does not get left lying around and if you leave cash somewhere you basically forfeit ownership of it since that's the cultural norm since ownership can't be proven.
The specific facts and details matter very much. Time and attempts to find the property by the owner are particularly important. There's a very big difference between leaving your phone at a bar and leaving your phone at a bar for a week, in the latter case the bartender has a nice new phone they can use.