From what I've read, the trigger for the mode switch was very detailed and narrowly tailored to the EPA certification testing, and included barometric pressure as a factor.

That makes it quite a bit harder to believe that whoever implemented it thought it was for some legitimate testing. For testing you want a trigger that is hard for anyone to hit accidentally, but easy for people who know about it to hit. You would not include barometric pressure, because that narrows the ability to get into the test mode way too much.

An ideal sequence would be some nonsensical sequence of inputs, like a specific sequence of left and right steering inputs, with a specific sequence of turn signals (often opposite of the direction turned) if the ECU has turn signal data available, interleaved with a specific pattern of taps on the brakes.