Could you please shed some more light (not necessarily the whole million candle power) on what kind of torch you used and where did you get it? Also, what version of W did you inspect that way?
I used on one occasion a cheap DIY-construction-building instrument called wire/stud locator. In “stud” mode you press it against a surface, calibrate with a small knob until it stops producing a sound, and then when you slide it around it will beep again if it finds a difference in material thickness. I don’t know how reliable and precise it is, but on that occasion it solved my problem. Your systems sounds better.