I'm pretty sure they don't need any excuse at all. They can just do a "random" screening - which is designed exactly for that.
The only reason to keep screening him over and over is if the detector really did find something, otherwise they are just wasting their own time because of a nebulous feeling.
Uh... The person has already exhibited enough suspicious behavior and circumstantial evidence to get frisked & questioned in a back room and their ticket cancelled. Indications are this is the kind of person being looked for, and if is, at this point is trying very hard to not reveal any further evidence of ill intent. Time to crack the egg and see if it's rotten.
The only reason to keep screening him over and over is if the detector really did find something, otherwise they are just wasting their own time because of a nebulous feeling.