However, I guess individuals in the police force might not be trusted with that kind of information.
Yes, there are both the options that they don't understand the value a paper wallet
can have and not take it seriously (throw it away; 'Nobody will come asking for some piece of paper') as well as the opposite: taking out the funds and being able to plausibly deny it: the finder could have unsealed it, the original owner could have moved the funds, someone else with a backup...
Plus, a note could be left which doesn't give too much information away to random people who happen to be passing by.
Exactly; you can quickly create a new email address just for that purpose, for instance.