Pilot reported being advised after landing that the aircraft had damage to the exterior from an undetermined cause.

Narrative

About an hour after arrival in ZZZ; I received a call from the Flight Operations stating that Maintenance had found damage on the fuselage after our arrival; and asking if I had recalled any late bags or anything of the sort.We did not have any late bags; and although I performed the walk around I was on the jetway and in the gate area for approximately the last 20 minutes prior to departure in a customer facing role. During that time there were a large number of gate checks that were brought to the airplane using a top of the jetway elevator style" cart. I think the cargo door was closed before I returned to the flight deck.I walked over to the gate and spoke with the Captain who had been scheduled to take the airplane outbound. The dent was found just aft of the nose wheel on the right side of the aircraft. Although I don't recall seeing the dent; I think I recall that the ramp agent was pulling up the belt loader as I approached that area; and it would have been right where I was leaning down to inspect the nose wheel; oxygen blow out disk; and underside of the aircraft. I looked at the damage from the next jetway over (and a photo from the next Captain); and don't recall seeing it on the walk around. It appears that it could have been approximately the right height for the top of the gate check bag cart; but nobody advised us of any contact with the aircraft or observed damage prior to departure from ZZZ1."