Since it hads been loaded at origin and not at this point - someone inside told someone outside. You don't make a smart strike like that on the oft chance of getting cash rather than fruit.
Here are some others for the list (dates may be trial rather than crime):
So, obviously, all of us should get out of the pax terminal and over to the Cargo
Looking at just four in 15 years, it looks like insider dealing is still thought worth the risk. My cynical guess is that LHR Security have not been given the resources to prevent this.