Well, obviously. What I'm saying is, why isn't there a sensitive enough metal detector which outputs a binary value of whether or not a dangerous level of meta

Well, obviously. What I'm saying is, why isn't there a sensitive enough metal detector which outputs a binary value of whether or not a dangerous level of metal is present? You need to check for two things I imagine: ferrous objects which could be pulled by the magnet, and conductive objects which would heat up when absorbing the MRI RF pulses. Both seem possible to do with a bedside device.