As I mentioned above, if you check-in online, print your boarding pass and have a cabin bag only, then the first time the airline would know that Squawk 6042 is ‘in the building’ would be during boarding at the gate.
However, at for example Manchester, you have to scan your boarding pass to enter the security check area. I wonder if that is just a check on boarding pass validity to keep non-travellers from getting airside, or is the scan also linked to the airline IT networks so that at that stage you are at least flagged up on the computer as being present? Same for if you use a lounge?