Probably someone else will say this also, but I was always taught not to leave things where they might tempt someone into theft.
Same reason you destroy sacred objects—not to tempt anyone into a sacrilegious act.
Cops were wrong.
Also, they were kids and maybe some of them not very smart.
I was put in situations like that at least 5 times that I can think of: my job as cashier when I was 18 (I was bonded, which helped me get another job later), a folder left right next to me when the cops all left the room, postage stamps left out at a campaign desk, an important phone call that came to me “by mistake” with both sides of the conversation audible at the office, someone trying to trick me into taking illegal cash for my political group.
That last one almost got me, because I didn’t know it was illegal. Fortunately, I was mad at the person who wanted o give it to me.
Sometimes paranoia, or just plain common sense and a knowledge of the way the world works can help you, so I guess it did. I didn’t fall for any of those tricks.