This is either the work of a ghost, a scary clown, some mutant turtles going through puberty, or an air bubble. Denver Water thinks that, likely, it's the last one:
@nexton9news
Stuff floating lids on sewer lines? Yeah, one year ago I was walking by some of that. Sadly it was not an air bubble. Called it in. Found a little root ball blocking the sewer line. Kinda the last day before we all started masking up.