Well... siphoning is nothing new. Last spring I almost blew up gas station with my boat. When I started fueling it for season opening, the gas started to run out the bilge drain hole. Turned out that in the storage somebody disconnect the tank hose trying to suck the gas and didn't connect it back.
I had less than 5 gallons in 55 gallons tank, so I don't think they were able to suck anything.
On the second tough the boat have identical screw-on caps for fuel and sewer tanks. Both of them have been loose and sewer tank was 1/2 full.