Cerebus

Cerebus is a Pit Bull program with the ability to fire Hellbolts. It will detect and follow a Netrunner. If the Netrunner escapes. Cerebus will lurk within the borders of the Net subgird in which it was launched and await the Netrunner's return.

Note: this means it could be waiting outside a particular datafortress; be on guard!

Source

 * Rache Bartmoss Guide to the Net, pg.148