Constant Firewall Messages in Console: How can I find the source?
The command "arp -an" will list all the MAC addresses and IP addresses of all the devices seen on the local network.

Once you have the MAC address, you can look it up to determine the manufacturer of the device, which might give you a clue as to which one of your toys is talking to your machine.  One such lookup site is here:

http://www.coffer.com/mac_find/

UDP Port 8612 is sometimes associated with a Canon printer driver.