Someone is stealing my petrol?

the bike is kept right at the back door in my garden,i do have security lighting but that's not deterring them,i also have it alarmed but it's not sensitive enough as all they have to do is pull off the rubber hose that goes from the tank to the carbs that's how they are getting it so they are cheeky enough to be that patient in waiting for it to drain that way,anyone have any idea's.