Sidewalk-Roomba is what I was picturing in my head, and their design has been a walk-ing exercise I do when walking around the city... basically I just observe the city as I walk through it and think about what it would take to make these feasible.
I was picturing stations where they would go to dock and dump and reload. You would have them assigned to a particular block radius and would monitor the conditions of each given area.
Perform cleanup and what not.
I was picturing something with a bunch of sensors on it, a clean/dirty water tank, tankless water heaters and a preassure washer with a battery pack.
The sensors could track the amount of foot traffic they see pass by them at all times - and you would know the ebb and flow of foot traffic throughout the city as well as the available walking space, timings of lights and buildup of foot traffic for a given intersection for each hour... lots of data.
[0]: https://www.npr.org/2018/08/14/638512558/nope-those-arent-ma...
(actually what spurred me to write was the surprise that the Guardian, a UK newspaper/web site used the American spelling for faeces).
https://en.wikipedia.org/wiki/Motocrotte