It was a sting operation where an grandmotherly type TSA shilling as a passenger wandered towards the gate's boarding door and eventually boarded the plane without a boarding pass.
Please note that the Gate D12 area is very poorly designed with unblocked access to the jet bridge door that is also obstructed from the GA's view by a wall & its placement behind & off to the side of the GA desk.
fwiw, this was a United Express flight operated by Atlantic SouthEast Airlines.
As with most IAD-LGA flights, this one was oversold, had confirmed & standby passengers waiting & the regular assortment of issues that totally occupy a lone GA.
From what I overheard, TSAGranny walked towards the door but when the GA first made eye contact with her, TSAGranny turned around & walked away. The GA then went back to boarding & trying to get the flight out.
With the GA's facing customers in front of her, TSAGranny walked across the large swath of floor space behind GA Desk/Wall & entered the jetbridge.
Then TSAGranny managed to waved a piece of paper by the person guarding access to the plane and boarded.
It was a youngish TSA Supervisor/Shrew who then couldn't wait to announce "gotcha" and proceeded to harangue the GA before boarding had ended.
Consequently, the flight left late & I who had taken the bump, was also forced to wait for the TSAShrew to stop her tantrum before the GA was able to continue with her duties and issue me a new boarding pass for another flight. The TSAShrew 's unprofessional display lasted long enough to cause me to miss my next flight.
What was really strange was the TSAShrew seemed to only rail on the GA, repeatedly demanding her credentials & Supervisor but I heard no mention of any issue with the employee who actually allowed TSAGranny to board the plane.
The whole scene was disgraceful and given the poor design of that gate area,I doubt any lone GA would be able to adequately guard that door while juggling all the duties of that job.