Ah, the bank NPC is missing because of the holiday event. They've all been kidnapped or something, so it's not currently possible to complete the quest. It's frustrating and a little retarded, I know, but it is a seasonal event and should be ending fairly soon. I've seen that exclamation point in Silverspring, but I've never seen an NPC in the area. It shows up on all my characters, so I think it is just a glitch on the map.
Edit: I expect you'll be able to complete the bank quest within the week, according to
this announcement.