So if it sprays some sort of chemicals? Virus’?
Recall the dude that jumped the fence, ran through an open door, and the started running up the stairs?
A quad-copter of the size described with one of the micro video cameras, controlled by a smart phone could follow the same path. Imagine a directed small payload. Once it’s inside how do you stop it? Handguns? Brooms?
Ponder it having a payload of a dozen micro quads or the mosquito size drones currently flying in labs. Pick your poison.
Purpose of terror is to generate fear and uncertainty. Even with no casualties the objective would have been obtained. TSHTF and then what?