(Thanks to Cancerman)

"The OnStar system utilizes a GPS transceiver AND a 3 watt cell phone system. Even if the system is turned off, remote unwanted communications with the system MAY activate a transmission. The system cannot be turned off unless the vehicle's battery is disconnected. Vehicles with the OnStar system are believed to pose the same radio transmission hazard as ordinary cell-phones.
In order for explosive operations to be permitted, the vehicles must either have their batteries disconnected or they must be removed to a minimum distance of 250 feet (76.2 meters) from any area where armed devices are being utilized."

So I guess somebody had to learn this the hard way. [grenade]

Yup, Bill Malone of Transtar Blasting -  now known as Stubby Malone.