My "guess" -- and that's all it really is -- is that it's referring to a deficiency that was subsequently repaired. A bad caulking job that was caught during final inspection, for example, and the void indicator wasn't totally removed after the repair.
When I worked in manufacturing, anything that wasn't up to snuff was indicated with a red tag and the letters NFG, meaning No Freaking Good (to put it politely).
Geoff & Jill
& Sierra, the little white monster
2013 Ford F-150 XTR SC Ecoboost
2015 Jayco Jay Flight 24FBS