I don't know about buses, but RVs do. Some jurisdictions have been known to cite folks for leaving them dangling. It's a secondary cap, and will usually leak if the main valves are left open. On RVs, there are generally two separate tanks, one for grey water (sinks and showers) and one for black (toilet). Each tank has it's own main valve (either manually operated or via button/switch), but they dump through the same outlet.
So they can’t claim they didn’t realize they dumped. It had to be a deliberate act.