Oh, I'll just read and post the solution here after all.
Here's the solution in Indonesian:
http://tinyurl.com/8366ev8
...well, that was quite something. I think I like the riddle better than the solution. So many clues for the solution weren't represented in the story.
"1) The valet was genuine, he wasn't involved in this crime.
2) When the old friend borrowed his keys, he drove the car to an accomplice's house who also had his own Honda Jazz. Then he returned with the accomplice's Honda Jazz and returned Honda Jazz #2's keys to the main character.
3) At this point, the car in the basement wasn't the main character's, but rather the accomplice's. Meanwhile, the Honda Jazz #1's keys were being duplicated by the accomplice, as they planned to return the original keys and to use the duplicates to use the car.
4) They set it so the alarm would go off on purpose, therefore the security guard would borrow the HJ#2's keys, making the main character believe his car was still in the basement. This way, the time when the crime happened would be falsified, to throw suspicion off from the old friend.
When the guard turned off the alarm, it wasn't the main character's car.
5) When the old friend returned to the party after dropping off the main character's car, he also brought another accomplice with him. This one was the man who claimed to be another party guest. He borrowed his own keys, drove back to accomplice #1's place, and exchanged them with the real keys, as they had been successfully duplicated.
Physical evidence: If forensics were brought in, they would be able to immediately determine whether the underwear had been left in the car since college or it was just taken off/newly bought."
...wtf
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Proud ValHellen x Maha_Bufudyne shipper.