It was actually quite common across the catering industry a generation and more ago. If the individual operatives weren't doing it then the manager was. Bars especially, where putting your own purchased drink in, not ringing it up on the cash register (there were fiddles to make it look like you were), and pocketing the proceeds was rife. Simplistic mechanical cash register displays on the front could be wound back with a sharp pointed instrument.
If the steward was putting their own coffee into recycled Maxpax cups, it was likely better quality than what was in there first time round. It was an appalling coup by some salesman from Maxwell House to inflict that on rail passengers as "coffee".