Maybe so. The only person I know with security clearance says that when they have to renew it, people come and interview them, their neighbors, family members, etc. To go to that trouble without checking their internet history seems foolish, but that doesn't mean that they do it. (This is in the USA; I don't know what level of clearance they have, but I know they work on classified projects.)

They do the same in the UK but you can quite happily run anonymous accounts and be knee deep in anarchy and they wouldn't know.