"Cool. Then it's a date," Sans will say, giving Lambert finger guns, as he nonchalantly starts backing away without looking where he is going. It's not completely clear whether he is now being extra weird just to bother him, or if this is just his genuine behaviour. Probably the later, honestly.

When Lambert finally breaks eye contact with him he will abruptly disappear. He'll be back at the end of your shift, brosef.