I agree, that's very bloody basic. Unfortunately I can also see it happening. And somehow it did happen, because in the days between when he left it and the match, no-one saw it, reported it, and he didn't discover it was missing, or report it. Rags pre-match sweep was clearly also woefully inadequate, and regardless of any statement the security guy comes out with about how it's entirely his fault, whoever decides what the punishment will be for the rags won't forget that they still should have found the device before filling the stadium up, given the fact it wasn't properly hidden (Hence my earlier point about how a pre-match search will only ever be as good as is practical, I had no idea the bomb was just hung up on the back of a toilet door).
You can see it happening because you are making the analogy in your mind with someone forgetting to do something in a normal job context,not a security exercise.That exercise should not finish until ALL the devices have been located. Every time a device has been found it is radio'd to the control centre and that area isolated.One by one. This one wasn't even hidden it was taped to the back of a door the fact it took four days for someone to notice it was there is also very strange.