ok first of all you've declared that he's "harmless" like 5 or 6 times as if we're not getting the hint that he is indeed
harmless. problem is, it's impossible for us to accept that in a text-form description of the event over the internet, so some of us are probably going to continue questioning this supposed harmlessness.
having said that... judging by his behavior around you at work: scared to really converse with you outside of "hi/goodbye", nervous and uncomfortable. added to that the "horrified expression" on his face when he saw that you saw him driving by your house--i'd say there's one of two hypothetical scenarios going on here:
1. he could have social anxiety real bad
. his shy behavior in the workplace is a result of his wanting
to ask you out combined with the social anxiety symptoms waging war on his mind/body. he's fighting the SA all the way and barely
winning, but he's wondering if it's worth the cost, uncertain of his chances, just overwhelmed with uncertainty and the effects of SA symptoms.
what you should do in this case
(assuming you like him): ask him out for a coffee after work one day, do all the talking and make it sounds like it's just a hang-out thing and not a date. pick a time and place that's close to work so it feels almost business-related, and set it sometime within the next couple of days (if he has a whole week to build up anxiety he'll be a wreck and nothing positive will come of it). once you're at the coffee place or w/e say, in a light-hearted manner with a smile "so i saw you drive by my house the other day/week, were you going to a friend's house or something?" then depending on his response, say "you don't have to be shy around me, you can talk to me anytime, etc.." try to instill a little confidence in him and see where it goes.
2. he could be an obsessive stalker.
be it caused by SA or some other disorder, he's losing
the war with his feelings, depression etc. but he still likes you and he doesn't understand why it's so difficult to be normal around you. he's damaged/traumatized from past experiences and doesn't possess the social skills to actually go out and get what he wants in life so he settles for imagining, obsessing, and fantasizing. you become an object to his desires, like candy and toys to a kid. He's given up on actually scoring a date with you and is settling for small tastes here and there: driving by your house to be near you, saying only "hi" to you at work just to hear your voice directed toward him. just small samples now and then so he can feed his adverse obsession.
if this is the case
, then i'm not sure what you should do exactly. i would say try to ignore him and just be cordial with him at work. wait for him to find another obsession.
i know scenario 2. sounds a bit extreme, like something out of a murder movie, but i'm throwing it out there based on his "horrified reaction" to your catching him driving by. probably isn't the case, but if it continues happening like more than 3 times like Aussie_lad said, then you might have some cause for worry.
(wow, long post