Where did it go...

I've been uber-focused on politics as of late, so I'm going to take a break for a second, and tell you what's going on in my life.
A couple of weeks ago, I drove to a party at a friends house, partied through the night, and got a ride home when the party was over; leaving my car at my friends house overnight. I got up the next day, and remembered that my car was not outside, and decided to ride my bike over to my friends house to pick up my bike. I get to my friends house, hang out for a while, and then leave my bike behind and drove the car home.
Now let me give you a little background on my friends house. This is a house in which four girls live; they never lock their garage, they never lock their back door, and they have a lot of lawn furniture and two bicycles that sit unlocked in their unfenced back yard. Not to jink them, but these girls have never had their house broken into, and have never had anything stolen from their back yard or garage. So, I'm figuring it's a safe bet to leave my bike in their backyard, so much so that I don't even lock up my bike (my fault) for a night.
Well, I get a call from my friend the next day, and she asks me if I had come that morning to get my bike. I tell her that I had not, but why was she asking. She said that the bike was no longer there. I really didn't think much of the comment, and figured she just wasn't paying attention, or that one of the other ladies in the house had moved it.
I went for a run later and ran over to the ladies' house. I arrived, not really thinking about my bicycle, but more just hanging out. I hung out for a couple of hours, and then when I was ready to head home, I went to the back yard to search for my bike. Well, lo and behold, it was gone. I searched the garage, searched every corner of the yard, looked in the basement, and checked in with all of the housemates; no dice. I was baffled. No fence, garage unlocked, lawn furniture, other unlocked bikes, and none of them were touched; my bike was the only thing stolen.
I'm not hugely upset about the actual bike, it wasn't terribly expensive or nice, but what frustrates me is that a) a free, energy-efficient, exercise-inducing, (almost) carbon footprint-free mode of transportation was taken from me, and b) the fact nothing else was taken (I'm glad nobody else lost anything, but I don't understand it), so it felt like a conspiracy.