He sets them off all the time.
He will be able to tell you the ones that work and the ones that don't.
Re: Hit and miss - we have numerous brands and varying quality ones.
They all seem hit and miss - to the point where I haven't used one for years now.
Keep it in the van, somewhere.
One of those special strong magnets might do the job. Would find where the plasterboard screws or nails are. Come to think of it, a magnet out of a magnetic filter might work.
I have one of those cheap stud detectors that I bought years ago that has a hole in it for your hand to grip it. I found it to work alright if I took the patience to slowly repeat testing each stud area.