Well I tried ot gently ask DD about blank pages (also Edexcel higher). GIven this was after she had come in and said it had gone really well, better than the other maths papers, I didn't want to deflate her by saying there might have been an extra question on the back that she missed, just said various people were thrown by there being blank pages in ithe exam She also didn't remember any specifically, but she did say that whenever they do mocks, they have these blank pages in which is apparently because of the "machines" that are used to print the real things, and they give them to them like that at school (and she didn't approve as it was such a waste of paper and the school didn't have "machines" and could have printed them as normal). DD as you might have gathered is a bit of a save the environment freak (we had whole issues because she didn't and doesn't want to print out practice exam papers because of the waste - but luckily DS has a brand new laptop in preparation of going to Uni in September (he took a gap year) and that has a pen, and she has been doing past papers on DS's new laptop with said pen - otherwise we might have had no past exam papers done at all!).
So DD claimed that she is well used to blank pages in the middle of exam papers, and she is sure she checked everything and checked she had found all the questions. So maybe some of them were printed with blank pages and some weren't, or maybe DD and your DS2 were just accustomed to there being random pages in the middle and didn't think twice about them, and didn't treat them as blank but as "machine pages".