I can pick upwall wart at 40' (just tried it). I have used it on some panels, as follow-up to last IR survey of the plant I work in. And have replaced panels, because of it. Haven't tried it on forced air or ducts at this point.
While it is a useful tool, it just gives the indication that there may be an issue, it is up to me to follow the proper procedures to complete an investigation and resolve the problem.
I have a panel to change, as i think the buss is getting bad. Same hot spot as in the last IR survey, new breakers were installed. Perhaps someone didn't look close enough, when changing them. Not critical at this point, but deserves another look.