Security...
Having had several conversations with corporate security at BA in a previous role, they have the means to install portable CCTV, including on board, if they need to get proof. This was done years back when trying to find out who was pilfering from baggage, and i believe also from onboard. Both very successful. They WILL easily be able to get a list of suspects based on who had access to the aircraft, and if they see patterns, you can bet it wont be long before they catch them..
There are actually very few people small minded or malicious enough to do such things, even though some might like to make noise about doing such things. And sooner or later, they will get caught, or someone else will turn them in.