Looking for a freshly booted headless Raspberry Pi on your lan?

Do a scan to prime your arp cache (nmap will do nicely) and then:

arp -an | egrep -i 'at (28:CD:C1|3A:35:41|B8:27:EB|D8:3A:DD|DC:A6:32|E4:5F:01)'

will find you all Raspberry Pi OUIs and their IP addresses.