Earlier quoted context omitted.
I'm guessing they're doing it during state emissions certification. When they do those tests they hook up to your OBD2 port [1] and generally a tube goes onto your exhaust. It'd be pretty trivial to detect, "Cable is hooked up, exhaust has more back pressure. Tune engine to X mode." 1: https://www.progressive.com/answers/what-is-car-emissions-te...
I thought they went away with the tube testing once the OBD data got good enough. Trick to passing the tube tests was to take your vehicle on a nice long ride to get hot (especially the cat). What I disliked about state emissions testing is that they didn’t do analysis/sampling of data to take an evidence-based approach to target testing against vehicles that tended to fail.
The fundamental problem with all the emissions testing is the gap between "as it's supposed to be done" and "as it's actually done." The people actually doing the work tend to be, more or less by definition, "car people." And they reasonably enough tend to avoid the more "engine abuse" ends of the test spectrum when they can do so - which does reduce the effectiveness of the tests in some cases.
For older diesels (my '97 7.3 Powerstroke, for instance), the "snap test" is supposed to involve flooring the throttle and letting the engine bounce off the rev limiter, while the sniffer sees how much smoke you emit (with a comically high pass limit - something like 40% opacity, which is really "a very dark exhaust plume"). The one and only time my engine has seen the rev limiter in my car was when I emissions tested it shortly after buying it, and did the procedure as written. Every other time I've tested it, someone else has run the test, and they'll bring the engine up quickly (but not floored) into the test RPM range, which doesn't involve going to redline - think 1800-2000 RPM instead of 3500 RPM. They way they do it, you pretty much can't fail if you drove the truck there, unless you've got a check engine light glowing or obviously missing emissions control equipment (no bulge in the exhaust pipe where the cat should be).
Because the people doing the test, very reasonably, don't want to be responsible for older engines throwing rods out the side of the block, and there is zero reason to have a lot of the older diesels at redline, no matter what you're doing (I'll let the engine spin to 3k when I'm pulling a heavy trailer up a long grade, otherwise I never even see 2500 RPM).
And there's also the various "ways around the test." A friend who used to have a rather fun to drive car noted that his sniff tests always resulted in the emissions profile of his mechanic's very clean, stock, Honda Civic.
There's only so much you can do, unfortunately, when people are motivated to go around the edges of the system. And I'm not sure "clamping down massively" accomplishes much at this point.