score:5
The French customs has a bunch of βbrigades de surveillance intΓ©rieureβ mostly in ports and train stations. I am not aware of any other location where they would perform a systematic airport-style security inspection but they are monitoring exits at other border points, including the nearby gare de l'Est. They wouldn't care about import rules for the UK (or be able to enforce that effectively) but about everything else (human trafficking, illegal drugs, weapons, etc.) They also have (or had?) explosive detection dogs that are occasionally called to help on completely unrelated suspicious packages on the Paris rail network.
That said, systematic checks are very costly and inefficient, whether on entry or on exit and the French customs doesn't rely on them so much so that's certainly an oddity that they would do it in this case. This report from 10 years ago suggests that back then they would catch someone with illegal drugs every week or so. Officers apparently complained that increased police and uniformed presence around the station meant that criminals were aware of the risks and that they had less and less to do.