Finding a phantom GCFI
Look around (usually physically nearby) for a tripped GFCI - that will be the one. If the circuit breaker is well-labeled (unlikely, IMPE, but in the ideal world they all are) you'll know what other rooms to check, as they would be the ones on that same circuit breaker.

Typically will be the other side of a wall, another bathroom nearby, out in the hallway, etc.