Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Distraction
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
Flight Crew Became Reoriented
Human Factors
We operated into DTW and had a little over an hour sit before our flight out. After we shut down; the FO got out to do his walk around; while I escorted an unaccompanied minor to the top of the jetway. After that I went and grabbed a cheeseburger. When I got back to the plane; the only people on board were the FA and a Check Airman who was about to do my line check. Both were seated in the back of the aircraft. I went and sat in the main cabin as well.  After a few minutes; an unidentified individual came on board and said we had an engine running. When I checked; the number two engine was running near idle. I shut it down immediately.
Air carrier Captain reported leaving the cockpit after parking the aircraft and realizing later one engine was left running.