HardwareGrowler doesn't notify bonjour service discovery, and as The_Tick pointed out, neither does Growl itself. It must be another application which uses Growl to notify of Bonjour related events.
How many apps do you have registered with Growl? i.e. how many apps are listed in the 'Applications' tab of the Growl Preference Pane?
Is there any icon with these Bonjour notifications? What is an example title/description of one of these notifications: that might help track down the culprit.
Just a guess, but are you in a LAN of many computers? Googling "Growl Bonjour" http://www.google.com/search?client=safari&rls=en-gb&q=growl+bonjour&ie=UTF-8&oe=UTF-8
suggests that it might be CyberDuck or Adium (just a guess - don't put too much weight on that.)