Re: I'm really trying to understand something.
1. Depending on the location, radar may not be an option because it'll interfere with other radio operations. Also, these things are pretty small and usually made of plastic with a reduced radio reflection so may not trip most radar systems without raising the risk of false positives (think of the thing going off when nearby birds fly past).
2. Programmatic one-way drone flights will leave no radio signature. And if the transmitter is on a mobile base, or multistatic, it'll be hard to pinpoint.
3. A coordinated effort can distract the guards (say start a fight) while the payload is dropped off and secreted away.
PS. To the guy who suggested a truck-mounted trebuchet, something that big will be hard to erect, take down, and/or disguise in a hurry (the moment you launched, odds are someone will notice you and jot your plate). Also, the net can be very flexible and made of strong cords that give some, allowing it to absorb impacts and render it resistant to blunt-force penetration (the whole rig would give first before it broke), leaving direct cutting or going over, both of which have countermeasures against one-shots while a drone can perform a sustained effort.