Well I know a thing or two about trained service dogs so......
Why would you think that it wouldn't be a drug dog, now a days with the screening process unless it is a package of explosives left some where in the building, they have other more sensitive machines that you only need to pass by and it will detect explosives. Next time you go through screening look for the agent standing next to a machine near the front of the screening line, thats a explosive detector, you simply put a finger or hand down on the device or sometimes they wipe your hand and the machine does what it does and it can detect minuscule amounts of explosives.
Also, why wouldn't they care about drugs, if there are people out of control on a plane, that would make them care.
Also dogs can be cross trained for both drugs and explosives, just like apprehension dogs are cross trained for drugs also for police departments. I would fear that someone with Meth or cocaine that either takes it on a plane or accidentally injested it or contaminates someone else with it would get the TSA's attention pretty quick.