Well, unfortunately the conclusion is a little bit anticlimactic, given the set-up. But since people asked...
On Sunday, there was no small amount of phoning around, as room-mate tried to figure out what happened, and where his clothes were. By and large, the story gradually came out. Indeed room-mate did get shitfaced at the party. Aparently, there was a drinking competition. Such competitions have a bad habit of turning out nasty, especially when there are tequila shots involved. Needless to day, room-mate "won."
After his win, it became quickly apparent to all present that (i) room-mate needed to go home, and (ii) room-mate was in no shape to drive.
So the logical decision was made, and one of the other party-goers shoved room-mate into his car and drove him home. Room-mate thanked him by puking all over the inside of his car. This is all, I suppose, pedestrian enough.
Where things take a slightly more interesting turn is when one realizes that driver has never been to our house, and doesn't really know where room-mate lives. Room-mate is of course not really in any shape to give this information reliably. So after some muddling around they finally stop in front of a house that they believe is ours.
Driver walks/carries/drags room-mate to the front door, which room-mate manages to open, so the assumption is made that it's the correct house (we conclude that the door was probably unlocked to begin with). Room-mate heads to bathroom and continues his colourful cleansing.
It is at this point that house occupant #1 comes out of his bedroom, sees driver and asks who he is. Driver points to bathroom door (from behind which are emanating all kinds of noises one expects in this situation), says he's a friend, that room-mate (who was presumably not mentioned by name) had too much to drink, and that he drove him home. House occupant #1 accepts this, and returns to his room. Room-mate finishes making a mess of the bathroom and heads into the bedroom to go to sleep (it should be noted at this point that the house has essentially the same layout as ours, facilitating this sequence of events). Driver figures his duty is done, and drives off.
Several hours later, house occupant #2 gets home, discovers room-mate, naked and covered in vomit, sleeping in his bed. "WHO THE FUCK ARE YOU AND WHAT THE FUCK ARE YOU DOING IN MY BED?" et cetera. (Rather rude, don't you think? Can't imagine why he was so upset...)
Room-mate isn't really sure where he is or what he is doing there, and indeed has at best a tenuous grasp on the principle of gravity (to say nothing of the direction in which gravity is acting), so of course has no good answer, and is summarily kicked out on the street, as-is. Room-mate wanders around in a daze for a while, then realizes he is in his front lawn (it later transpires that driver got it almost right; the house where he dropped room-mate off was only two doors down from our own), so rings the doorbell. I answer and the rest you know.
* * * * *
Room-mate's car was still at chez party, so no big issue there. House occupant #2, after kicking room-mate out on the street, apparently called the cops who (not arriving until after I brought room-mate inside), took room-mate's clothes back to the cop shop. Shoes and jacket were salvageable; pants, shirt, etc. not so much. Wallet, keys re-united with owner, phone no longer functional. Oops. Room-mate not too happy about prospect of getting another one, but what you gonna do?
Moral of the story: tequila shots + drinking competition = making all kinds of new friends.
Why we didn't bother with Christmas lights this year.