I'll jump in and play the guessing game, too.
Most of these measuring tapes have paint and a mylar coating that provides a minimal amount of electrical insulation, but the edge of the tape sees a lot of abrasion so may have exposed metal.
I'll guess that something was plugged in, and someone was measuring the length of the wall across or above the outlet.
The metal rule of the measuring tape slipped down between the plug and the face of the receptacle. The edge of the tape, which probably had bare metal, contacted the hot and neutral prongs, resulting in an arc flash. (The sharp line across the top of the holes suggests this.) This alone would not have tripped the GFCI, as the current between hot and neutral would still be balanced. Also, if the initial short was to ground, the GFCI may have tripped before the current was sufficient to create an arc flash.
However, the initial arc flash melted the coating from the rule, exposing more bare metal. After the initial arc flash, continuing motion of the tape (or reaction of personnel) caused the bare metal to contact the cover plate while the edge of the rule was still contacting either the hot or neutral prongs, resulting in a current imbalance between hot and neutral that tripped the GFCI.
Yes, it could happen in a properly wired outlet. Something similar happened to me when I was about 6 years old. I was playing, hanging metal coat hangers on an electrical cord that was draped from a receptacle to a table lamp. I then hung a coat hanger on the plug, where it promptly fell into the gap between the plug and the face of the receptacle, causing a short circuit, an arc flash, and a tripped breaker. (This was not a GFCI protected circuit.) I was a fortunate child, and suffered no injury.
(On deeper analysis, the blade of the tape contacted the neutral prong or cover plate first. We can tell this, because the carbonation radiates from the contact point on the top of the hot prong. If it contacted the hot first, the carbonation would radiate from the second point of contact when the circuit was completed. This also explains where there is no pitting on the cover plate; if indeed it was a short between the cover plate and the hot prong, contacting the cover plate first would ensure that the arc would form at the second point of contact, the hot prong.)