Maybe we need very clear, standardized signals for entering a surveillance zone (where someone not obviously present, possibly a corporate stooge or AI or simpler-than-AI automated detection system, is judging behavior) so that people realize that private behavior (like casually showing off guns to each other, safely, which I've seen done myself and was not cause for alarm—guns were legal) is no longer private.

The steelman -against- what waymo did is:

Guns are generally legal, even in California. This is America, not the EU. Nobody was harmed, nobody was threatened. This is like calling the cops on someone open carrying because you don't like open carry.

That is what a lot of pro-2A libertarians would argue.

The steelman -pro- what waymo did is:

Private conveyance. Even though the occupants thought they were "alone", there was someone/something else present, which felt threatened (or thought the passengers represented a potential threat to others) and called police, as most cabbies or uber drivers probably would.

I don't think there's a right or wrong answer. This is a grey area, and I think it'll likely resolve in favor of waymo, and it won't really bother anyone except the staunch libertarians. If the gun hadn't been illegally possessed, likely nothing would've happened after the cops were called.