I did a search and got this:
"Twelve states—California, Connecticut, Florida, Illinois, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, Pennsylvania, and Washington—require the consent of all parties for you to record a conversation.
However, all but 2 of these states—Massachusetts and Illinois—have an “expectation of privacy provision” to their all-party laws that courts have ruled does not apply to on-duty police (or anyone in public). In other words, it’s technically legal in those 48 states to openly record on-duty police."
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I think the key here is the word "conversation" in the first paragraph. So I'm thinking that silent recording is legal in all 50 states. Of course if I'm wrong I could be in for a beating.
I have a Samsung Axxion cam on the bike that they won’t see unless they’re really really looking for it.
I am never going to mention it, either.
I don’t look good with lead in my head.