That's the default allocation size.
Nothing to worry about. If you want to check it totally, see if you can find it in your Disk Manager.
In there, you can see partitions and unallocated space.
Just type
diskmgmt.msc
In your Run window. Or the Search Bar in Windows 7.
If USB-sticks are beeing regognized in there, then you can see what that space is.
Or you could try to reformat it - In Windows 7, I always tend not to use Quick Format.
Good Luck!