I'm not going to name the operator or the service, but last year I was on near the front of a bus where a passenger boarded, placed their £2 in the tray, and wandered up the bus driver could even issue a ticket. I could see the machine where I was sat. I'm very sure the driver would have issued a ticket if the passenger had waited for one. But the machine issued no ticket.
Not saying the driver pocketed that money because I don't know for sure they did. Maybe they simply chucked it in the cash tray. Who knows. But when there's cash fares paid, and there's no inspectors or inspections, and passengers who don't even wait for a ticket, well you can see scenarios where it could happen.