At CERN, if an unattended bag is found anywhere on the site, the Fire and Rescue service will be called to evacuate the area, maintain a security perimeter for as long as necessary, and attempt to identify the owner. If the owner cannot be found in a reasonable amount of time, there’s a very strong chance that the bag will be destroyed.
CERN's main campus is the size of a small town on the Swiss-French border, so there's a lot of opportunity to leave stuff unattended. In fact, there's an entire blog devoted to unattended chairs at CERN, and when I was there, I got pretty badly bruised riding an unattended shopping cart down a hill and into a wall. Neither the chairs nor the cart called for evacuations, but mysterious luggage might.
The statement doesn't specify whether or not unattended luggage goes into the Large Hadron Collider (it definitely doesn't), but it does remind visitors to make sure their bags are clearly identifiable. Or else.