When working for a certain airline this particular behaviour of stealing alcohol from the aircraft was rife. As it was a 'free bar' nothing had to be accounted for. So once the airline suddenly noticed that their aircraft were returning to port with an empty bar and very sober pax they suddenly realised that something wasn't quiet right.
Pilots and flight attendants would often walk off with nav bags clinking, full of wine, champagne and spirits. With all those bottles tucked precariously into those nav bags up front it is a wonder the nose of the aircraft could stay upright. I myself never took any, as 1. I am not a big drinker and 2. I knew that management would suddenly catch up, and that they did. (They ended up going to the extreme of checking rubbish bags from the aircraft on their return into port, and seeing if the empty bottles in the rubbish collaborated with what was missing from the bar). No wonder this airline went bust!
I have also heard of one particular pilot who was such a tight ar#e that they would steal.. wait for it... light bulbs and toilet paper from the hotels that we would stay in.. surely their wage was good enough that they could afford these particular items from a supermarket????