My niece was on an American Airlines flight Sunday from Durango, Co. to DFW. Not long after takeoff, the captain announced the plane was missing a fuel cap and was going back to Durango. The plane was a CRJ.
Passengers were de-boarded. After the missing cap or a replacement was found, the plane left again for DFW.

A very limited internet search suggests a missing fuel cap is not a major safety issue. Nevertheless, it apparently was big enough to force a turnaround.

Any experts out there with some insight on this?