Sadly, other than alot of barking, my dog would probably act like the Cocker Spaniel, running around in terror.
My last dog Angel, the shep/husky mix? I know for a fact she would attack. My cousin walked in the door one day since we had the main door unlocked. She rushed him and jumped for his throat, then kept going after him. Thankfully we were about 5 seconds away, because she probably would have gotten him on the ground if he truly had been an intruder.