My company bought a bunch of flash drives, 500 1 GB drives, to be loaded with marketing materials and given out at a trade show. While testing the loading of the drives I noticed that some of them have used disk space, around 25 MB. I have checked for hidden files and run scans, but nothing comes up. Is this just bad formatting of cheap drives or is there a security risk? After reformatting one, it showed 948 MB free space, which is what the majority of the drives are showing. About one out of six drives have some used space.
All file systems have overheads. These are usually displayed as used space. In your case it's formatted with NTFS which have significant overhead. It will get better if you format them as FAT or FAT32.