There should be a temp file in the same location as the file. The temp file will be named the same as the file but with a tilde (~) in front. If you open that file with Notepad or some other text editor, it will contain the user name of the person who has the file open. More likely, SW crashed and the temp file was left behind and the file isn't actually open.
Hi Sara, I'm guessing solidworks is getting this information from the "lock file". In the windows operating system, every time a file is opened, a "lock file" is created. You may have seen these before, they are hidden files that start with ~$. For sample, if you opened up file: part.sldprt, windows would create a hidden file called ~$part.sldprt
If you open one of these lock files with text editor (notepad or word) you will see that inside the file it has the username of the person who has the file open. This is likely where SW is getting its information.
If you have a file that is "locked" by another user and you are certain they do not have it open, you can manually delete the lock file and then the file will become available again. Sometimes this is necessary if a program crashes or is forced closed abruptly and the program is not able to free up the file. To view lock files, you will have to turn off the windows folder option "hide protected operating files (recommended)" and turn on "Show hidden files, folders, and drives". then refresh your folder view.
Hope that helps,