This may not look like what you expect a metal detector to look like, but its small and easy to keep in the car in case the need arises. 

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I mainly use a metal detector when I find horizontal cracking in walls to see if the cracks are in line with where I would expect wall ties to be - horizontal cracking every six courses is a good tell tail sign of walltie failure. If this cracking matches the metal detector's beep, its nearly a certainty. 

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I also use it when inspecting damaged areas of stonework, mainly details around things like bay windows and crenelations as these are sometimes pinned together with iron bar or dog cramps which when exposed to the atmosphere, can corrode and shatter the stonework.