TOO LOW FLAP important consideration
TOO LOW FLAP is not simply restricted to final approach with the final landing flap not yet set.
With only approach flap set but otherwise in the landing config, it is possible to get this callout with steeply rising terrain, more so with normally expected higher downwind groundspeed during circle to land. Less steeply rising terrain could still be a great danger, but there may be no TOO LOW FLAP callout due to reduced rad alt ROD. Most other GPWS callouts are inhibited because of the aircraft configuration.
Check ALL the conditions that can give this callout.
There are many cues in aviation for immediate action. There is a good case for immediate action in this case.
The most modern GPWS may have addressed this.