I haven't seen any PNC branches around Grand Rapids posted. I have heard of some banks asking customers to leave, etc. though. In my 8 years of CC/ OC in Michigan I have only seen 1 sign on a building banning firearms... and it wasn't a bank.
Edit to add: Check that... I'd see a sign if I were to go to the Grand Rapids Gun show on 28th Street. The company that runs the one at the Delta-plex allows people to OC or CC, however.. Strange/sad/unbelievable that the only place I'm likely to see a sign is a place considered a "mecca" for local gun owners.