Any parcel must display the complete correct mailing address of the recipient. Describing a building on a street or expecting that your postal carrier will know the apartment number of your friend when all you can provide is the address of the building… this will not result in your mail being delivered as hoped.
Tampering with the mail is a Federal crime. If you see someone other than a postal worker taking anything from your mailbox or a person doing something suspicious with a post office drop box, call the police immediately. Most people who steal mail do it for the purposes of stalking, fraud and identity theft, so it’s not something to turn a blind eye to. Some people mistakenly believe a thief can’t cash their paycheck or get information that would allow a thief to fraudulently open credit cards in their name or fraudulently use their Social Security number. A thief could also re-route your mail by fraudulently submitting a change of address form while pretending to be you.