Some one will find it or notice someone is missing. However you can get (or make) holders for (blank if you must) cartridges with a spring driven firing pin released by a trip wire. The unexpected flash/bang would be a brown trouser job I would have thought...
Technology to alert you that something/one is present is one thing but how about making access difficult in the first place? Can you plant a nice thorny hedge alongside the existing fence, attach coiled razor or barbed wire to to the top?
You say you have fencers, wire one to the container? or to something you have to get past/climb through/over to get to the container. Might be worth taking advice on what warning notices would be a good idea to have about. Otherwise some scroat will try and sue you for damages...