I have an old Zircon 5.5 stud finder, it does metal, electricity and deep scan. Works well on tiled walls and plaster. I occasionally get a hold of old fir posts and or beams sometimes painted, a couple times I cut through nails that rusted off over an inch deep and then were painted over for many years, (no rust stain). The scanner never caught them.
The same scanner will pick up a cut nail sticking out the middle of a real 2X4 inside an old style plaster & lath wall.
Never bite the hand that looks dirty!
The more you know the more you're worth