B737 Captain reported a deadheading pilot observed the left-wing leading-edge panel inboard of the engine bowed up in flight. Maintenance removed the aircraft from service.

Narrative

After arrival at destination a deadheading pilot stopped by the cockpit to show us photos of the leading edge sheet metal panel inboard of the left engine. Five rivets in a row appeared to be missing causing the panel to bow up at the location of the five rivets. After landing the panel returned to its flush position with no evidence of any problems. I notified MX via Electronic Log Book (ELB) and spoke with Maintenance Control. The technitions climbed up on the wing and could not find any problems. After showing the the photos of the bow in flight it was discovered the the rivets were still in place but the rivet heads had pulled through the sheet metal. Without the inflight photo evidence; this problem would most likely be difficult for maintenance to discover without taking the wing apart. The problem rivets; while still in place; were hidden from below by the LE heat duct. The aircraft was removed from service and we took a delay waiting on an inbound aircraft to fly our return flight to ZZZ. This makes me wonder how many other rivets may be exhibiting similar behavior but go unnoticed.