DHC8 Captain is informed after block in that the right inboard main gear wheel and tire is no longer firmly attached to the aircraft.

Narrative

We taxied out and took off normally. We flew a normal flight and landed. Upon shutting down the engines and completing the parking checklist a Ramper knocked on the side of the plane and asked us to come out and look at something. The First Officer went out and looked and told me to come see. The inboard tire on the right side of the gear was missing its hubcap and was essentially hanging onto the axle. It looked as though the tire and rim had removed itself from the axle and were just hanging on. I went back into the plane and starting writing up the issue while the passengers deplaned. A passenger; as she was deplaning; said that she saw something fall off the plane during takeoff; but figured it was no big deal. Had we been made aware of this after takeoff we would have returned. We then finished the write-up and called Dispatch and Maintenance. Also; the First Officer sent a picture using his phone to Dispatch. We had heard a plane about five or more minutes after takeoff from Cleveland report debris that they thought was a hubcap on the runway to departure. Looking back it must have been ours. There was no way of knowing it was us; the gear was safely up and the takeoff was normal with no abnormalities. The First Officer performed his walk around and nothing was amiss there. Obviously; if there was some indication that we thought there was a gear problem we would have returned.