Drones do not necessarily require large production facilities and can be launched from a truck, making them an elusive target.

In other words with just the parts for a drone it is theoretically possible to assemble a drone anywhere even in somone's garage or warehouse anywhere. As long as they had a full set of parts it could be assembled anywhere. Also, they use wooden propellers so splinters go everywhere from the propellers when they explode sort of like wooden fragmentation from some kind grenade or something. 

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Tracking Iran's military attacks on Gulf states: NBC NEWS:

 

Former military officers and experts say Iran’s missile program will likely be crushed if the air assault continues. But they say wiping out its drone capacity is a more difficult objective. Drones do not necessarily require large production facilities and can be launched from a truck, making them an elusive target.

“The challenge is it’s probably relatively easy to hide these things, and so finding all of them, bombing all of them, is going to be hard,” said Tom Karako, director of the Missile Defense Project at the Center for Strategic and International Studies, a Washington, D.C., think tank.

“It comes down to how good is the intelligence, ours and the Israelis’, in terms of where everything is,” he said.

Iran’s ability to keep up missile and drone attacks has raised questions about the effectiveness of the U.S.-Israeli air campaign, the quality of the intelligence underpinning it and the assumptions that shaped the assault, according to former officials and analysts.