I got this when entering the Commons (at the invitation of an MP, but not in his presence) a couple of months before the occasion I described when I wasn't searched at all entering the Lords. That's why I remembered the distinction so clearly.

Perhaps I am asking a stupid question, because on the occasion I wasn't searched I was actually entering with the earl who was my host. Maybe it is still the case that there is no security check for those entering that way! Using the Peers' Entrance, I could easily have hung an exploding bag from Lady Thatcher's coat-hook.