My grandma used to work at a casino. She would turn in things she found and the people running lost and found would always say it was claimed. Then she sees a ring she turned in on one of the managers fingers about 60 days later. After that she “never” found anything to turn in.
Unfortunately trust and doing the right thing is going by the wayside!
Well, when it comes to the potential of the police doing that, .... then at first glance, it would appear to low-down-dirty-rotten, like your example.
But the more I thought about it, I realize the ease-to-which the police might not think it's too "low down dirty rotten". Because :
a) IT WAS NEVER YOURS TO BEGIN WITH . You only found it. So how have you been harmed ?
b) You turned it in to the police dept. knowing FULL WELL that it might be claimed.
c) They have no obligation to tell you who claimed it (privacy laws)
d) Therefore, on the 29th day, if no one has stepped forward to claim it, then .... you can see how a non-reputable front desk clerk and the police dept. lost & found might just adopt it.
e) Anything not claimed by someone in the public, nor claimed by the person who turned it in (ie.: come in 30 days later to claim it, after paying storage fees) is warehoused at the police station for eventual police auction. And those auctions generate a pittance of income (for routine items like rings, watches, etc...) to the police or city.
And so you can see how some police persons might just figure "no harm no foul". Yes it doesn't make it right, but .... you can see how someone with loose morals could justify it in their mind.