But I personally haven't heard of any other cases when this isn't considered a thank you gesture
It could also be used to call another driver's attention (e.g. if you have a broken taillight and did not notice), or to warn against a police trap (in the pre-Waze era) and maybe a few other things, but it never means "thank you".
Car coming the other way flashes and it means hazard ahead.
If it's at night and your DRLs are the only thing on it means "turn on your lights moron"
At an intersection it usually means "go" to whichever party should be yielding to the flashing party.
Of course if you're in the left land and flash your brights at someone who's clogging up the left lane there's a good chance they won't understand it because if they could understand the context they wouldn't be clogging up the left lane in the first place.
At least, that used to be the only time it was legal to use them here (Sweden). They recently added a rule that it’s legal to use warning lights while being towed, too. Either way, it’s certainly a traffic offense to use them while driving.