So now we know that these scanners fail in two major ways: In the Hamburg airport evaluations, there was a 54% rate of false positives. When they do alarm on something, more than half the time it's a false alarm. Now in the Northern Ireland prison evaluations, we see that 43% of prohibited items were missed. BBC: Scanners ruled out for NI prison searches after pilot Electronic scanners like those used in airports will not replace full body searching in prisons in Northern Ireland. They failed to detect nearly half of all test items during a three month pilot scheme. Items not detected included drugs, mobile phone batteries, scissors and a knife. Nearly 1200 prisoners and prison staff were searched using two millimetre wave scanners....A prison service evaluation report says the scanners detected just 57% of test items. The items not detected include a knife and scissors. Prisoners searched using the scanners had to volunteer to be part of the process, so all of the illegal materials detected during the test were carried by prison officers who agreed to be part of the trial.