I ended up just turning them on manually so really no big deal, but this is my first Tesla - does it need to be raining sort of hard for the wipers to auto?
The Auto wiper function is very... umm whats a word I want to use here... "sporatic" in its activation. That feature is one of the most frustrating ones in the car, over my 5 years of ownership.
To answer the general question, the auto wipers on a Tesla model 3 or Y do not use the industry standard sensor that basically every other brand uses. They use the front cameras to try to "see" the rain drops, as the CEO thought it would be cheaper and better that way.
There are a "lot" of threads here on auto wipers, sometimes they are better, sometimes they are worse, but they tend to not work very well at all in "misty" conditions since the cameras dont tend to see very small rain drops. Get comfortable with turning on wipers manually and changing speeds manually, would be my recommendation. I do that with voice commands (both turn them on and off, and change speeds).