Indoor street sweepers?

I knew that my new place of employment, aka The Center, was big, but it really hit home this evening how big. I'd recently wondered how the cleaning crew dealt with vacuuming the numerous large hallways of our organization. Tonight I witnessed their solution: mini ride-on street sweepers. They are massively quieter than actual vacuum cleaners, and they seem to do the job just fine. I don't know if they come back and vacuum the individual offices, or if they just do those 'on demand'. It was very interesting to see the vehicle making its way slowly down the hallway though.