Bicycle detail

I got home around 6:30 PM on Feb 23 and locked my bike to the bike rack in our backyard. Our backyard is fenced with a gate that can only be opened with a certain key (all of the maintenance and residents have the key). I locked my bike (just the frame) with a 6-foot long combination lock. At 9 PM I took the trash out and did not notice anything different, so I believe that the theft must have happened after 9 PM. At 9:15 AM I went outside to get my bike and it was gone - there was no trace of any part of it or any part of the lock, including no bits of plastic and metal. There were multiple people around at the time, but none of them noticed anything. Additionally, my roommate's bike (which was on the bike rack at the same time as mine) was not stolen; he could not recall whether or not he saw my bike when he left around 8 AM.