Can you smell weed lingering off of someone's hair (I know, weird question..)?

yes you can smell it. Even if you can't smell it others may be able to (you become used to the smell). The smell may have also been on something you were wearing/had with you. A jacket, purse, shoes, etc can carry the smell long after you've lit up. Hair styles may even keep it around. The smell could get stuck in dreads, or stick to hairspray, even stick to hair elastics!