That's an issue we have discussed earlier, and people handle it differently. I prefer to advertise as mush as possible. That's why I asked the Mapillary staff if there were any magnetic signs available to put on the sides of the car. Much like The google car looks like. Unfortunately not.
However, there is a PDF available that you can print out and hand it to people who ask what you're doing.
I will be doing parts of a walking trail here in west Sweden, that is ~200 km in its full length, this summer, so I guess I will meet a couple of wondering people along the way.
When it comes to sticking a 360 cam to the rooftop of the car, I'm still trying to figure out a good way to avoid as much of the car as possible in the nadir area. One problem I've stumbled into is how to connect the USB-C cable while using the camera screw. If not charged, I will only be able to shoot for ~30 min before I run out of battery. But I need some kind of special screw for that...