USPO procedure question, answer urgently needed

I lost a cashier’s check by dropping it into the outgoing mail slot at my apartment building by mistake. It’s in an unstamped sealed envelope with my address and unit number on it.

If I meet the mail carrier there today and ask for it, will s/he give it to me or would that violate some rule that they have? This morning before I left I also put in a note to the carrier describing the envelope the check was in and asking them to put it in my mailbox when found.

Call the Post Office. The carrier might not even see the envelope, let alone your note.