Here's a new one (maybe)
 

Going through a checkpoint in SEA the smurfs were asking that you show your boarding pass before going into the strip search machine. Anyone with a paper boarding pass had to show it but anyone with a phone one did not.

Was this after the document checker had already checked it?

Pretty sure it was.

What happened if someone didn't show their boarding pass to the strip search screeners? How did they know who had had a paper one and who had had an electronic one?

I had a paper one but I just told them I had a phone one because I didn't want to find it. Just found it to be another classically stupid tsa procedure. Even the smurf had a recognition of the utter stupidity at play.

Since you can't have anything in pockets going through the Strip Search Machine I normally stick my ID and Boarding Pass in my carry on after clearing the TDC. Doesn't matter if it is paper or phone I do both the same. I generaly have a paper version tucked away just in case my phone stops working.

It used to be the norm. Now, it's rare. If they do find anything in secondary, they will then locate the PNR information, even on a phone, as they write all of that into the log for transfer into one of their databases. Not really much of a big deal.

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When was it the norm?

Everyone I've ever seen in the NoS has been told to pose with empty open palms. When I get my gropes, I am told to keep my palms open and empty.

Was TSA making exceptions and allowing all those folks to get scanned or groped while holding their phones or their BPs? Or did the TSOs take the BPs/phones and hold them until the scan/grope was over?

Well at this checkpoint the key was to have nothing in your pocket. I was told everything in your hands and nothing in your pocket.

They can't locate the PNR info from all mobile phones where a mobile phone boarding pass was used with the TDC.

I've noticed that the procedure seems to be different at different airports. At some airports (DTW for example) TSA makes you hold your BP and ID in your hand as you go through the noodle-scope. People without at least an ID in their hand get yelled at. This was a few years ago before mobile BPs were commonplace though.

I used to get randomly told I had to show my boarding pass by the scanner entry smurf, but not in a while.

Guess it depends on the airports you fly through. I have not seen anyone allowed to have something in their hand(s) since the NoS was introduced.

I have seen guys try to hold on to their wallet for a grope or the NoS, but the TSO always took it away to run through the xray.

Ah, they're stepping up their anti-Marilyn procedures.

Though it's still not as bad as flying TATL out of Europe. I gave up playing the "how many times did people ask to see my passport and BP" game on my last trip, because I kept losing count.

Since you cannot have anything in your pockets going through the machines I always have my USA passport and boarding pass in my hand when going through security. I have always been allowed to do this.