My guys have the "Fluke" pen model...I still have my "Greenlee" pen, but I don't put all my trust into it...if I'm leery, I drag out my "Etcon" solenoid-type tester...there've been quite a few times that I've gotten false readings, and then I recheck with the solenoid, and there's no voltage present....

.."if it ain't fixed,don't break it...call a Licensed Electrician"