At night?

I doubt that. At daylight with normal background noise level, possible.

The modern razor blade based robot mowers are barely audible. Segway claims 58 dBA, for context. Anywhere near a city (which, these smaller robot lawnmowers are really meant for smaller city lawns) is going to have close to that for background noise levels. Maybe it's partially audible if you're standing next to it, but with fencing and some distance between you and your neighbor, you aren't bothering anyone.

Hm. With 58 dBA extra noise in a silent area you can bet, that I will be bothered.

In a loud city where it doesn't matter anyway, yeah well, who cares.