It's very easy to think of people as 'stupid' when you're only looking at prosecution evidence here.
* Note someone x-rayed the box to find it empty - obviously not just a screwed down lid that you could just undo and see nothing inside
* In a different report you learn that he falsified MOD documents in order to validate them
http://www.ibtimes.co.uk/articles/486141/20130703/gary-bolton-fake-bomb-detector-scam.htm
* Public bodies in some of these countries aren't necesarilly like the ones in the UK - there may be no joined up connections - security at airports will not have ready access to explosives to perform their own tests and may not be able to get help from their armed forces to perform tests on them.
Why would they ruin a perfectly good detector by cutting it open when they have documents supposedly saying how good they are from the UK MOD?
That's the thing about confidence tricks they seem so obvious when you know that they are.