Disapearing drtnk

Today I'm working at my wife's store. I just had a very unusual occurance. I cannot figure a logical explanation for. Having skipped breakfast to get here on time I fixed a sandwich from makings in our small store refrigerator. I poured a mug full of RC cola to go with the sandwich. As I sat there eating the sandwich I noticed the level of my cola had dropped a half inch in the glass mug. Thinking this is just something it do with the fact it was filled with a carbonated cola, I continued eating my sandwich; however, I did watch the cola in the mug and it continued to drop in the level. When it dropped about a half inch I picked it up to see if it was leaking. It wasn't. The bottom of the mug had a bit of condensation, but no liquid was in evidence. I decided to drink the cola while I still had it. Maybe I should have let it go and see how low it would get. The mug sat on a wood table. There was nothing close to it. The liquid had nothing to sink into. The mug was nor broken There was no obvious cola anywhere but in the mug. Where did the cola disappear to? Is thee a thermal explanation for it? Whatever, it was just plain weird.