Many years ago whilst serving in the RAF I read Ernest K Gann's book 'Fate is the Hunter'. In one of his stories he recalls an unqualified pilot who managed to get through the net and was only 'discovered' after flying for an airline for some years. (If you haven't read the book, you should - it's essential 'trade-training' for pilots.)
Years later, as TRE on an overseas operation and later CTC for a UK airline, I came across several pilots who somehow got through 'the net'. Whether or not they used P51 hours to get their licences I'll never know, but a simple 'sim check' may just have weeded them out. It's very difficult to 'dump' a pilot later, and there's a reluctance to brand anyone with a Fail Certificate. It's a funny thing, but I found that these individuals had, without exception, a very high opinion of themselves (over-confidence) and were reluctant to accept the 'soft' option of resignation.
rts