Just back from the budget weekend return on Ryanair. Security checks leaving Bordeaux... everything in boxes, scanned, walk through scanner..no problems. Return from Stansted wearing same clothes..set off scanner..full body xray in cabin thing, shoes off and x rayed, frisk search..turns out it was the decorative zips on my trousers! Interesting though that the machines are obviously different at different airports.
I am led to believe that the sensitivity of the machines can be increased or decreased depending on the level of alert. Clearly the higher the alert level the more "false" alarms that there are and the more security staff you need to deal with the necessary subsequent searches/scans, and to man additional lanes to allow for the slow throughput.
Even in the UK there can be differences in scanners and levels of security and some time ago I was pulled up at Manchester for having a small tube of ointment in my hand luggage which should have been in the plastic jiffy bag (something I had forgotten about and had slipped into the corner of the case in the folds of the fabric lining). This had not been picked up in France nor at Heathrow on the outbound journey.