Last time I did it, which was about 6 months ago, the procedure was that after otherwise completing the checkin procedure you went with a porter who retained your boarding card to the screening machine which was located opposite the check-in desks (i.e. by the wall towards the roadway outside). You (and anyone else in who wished to) could observe your bag being put through the machine from about 6 feet away. I recall that I was asked to open the case, and then stand back outside a roped off, but not screened, area while they looked through my bag. Again this process would have been visible to anyone who wished to observe. Once the search was complete I think I was allowed to lock the case again before it was taken back to a belt near the checkin desk by the porter, who then handed me my boarding card.

It is only at SFO that I encountered all this nonsense.