3GB of internal storage is lost

I have a Sony Xperia SL with 32 GB internal storage from which 25 GB is available for user. When I bought the phone, everything was OK. but last month I found out that there is only 1GB left free, when sum of the files in the memory was only 21GB. I deleted some files and reduced the filled capacity to 10GB. However, I still have only 12GB free on my device. It sounds like 3GB of my storage is lost. (I have checked LOST.DIR and, for those who want to know, I have considered the thumbnail files when I summed up the files)