re: when asked where are you going... that's totally up to you. it's really none of their business, but if you're feeling amiable, you may wanna tell them. maybe they'll learn a lesson ... maybe not. besides....anyone who bedgrudes you for moving onwards and upwards is no friend of yours...
echo brian...especially the part about making sure your pc is clean and you're ready to instantly leave. this isn't a big deal. you don't know the guy and, tbh, probably won't see him again, so who cares how he perceives you? just be diplomatic and professional - which it sounds like you...