Password protected workbook continues to be locked up by users

I have a question that I am curious if anyone has ever come across. I'm not sure if this is an IT question or an excel question. But I figured I would start here...

I have a large workbook (Excel 2010, .xlsm) that I keep in a shared file on our network. The file is password protected and only able to be opened as 'read only'. No other users know the password, so no one else should be able to open the file for editing. However, there have been a few cases where I try to open the file myself and I immediately get a notification that the file is locked by 'another user'. Each time that has happened I have had to have my IT group look on the server to see who the person is, which I then go and talk to, and find out that they only accessed the file as read only, and in all cases the didn't have the file currently open. It somehow got "hung up" on their profile. I have to have IT kill the file they show as open so I can access it.

Is there possibly something in the way I have saved or secured the file that could be causing this? Or is there a different way I can secure the file without having to password protect it like this? It's not necessarily for true security reasons, it's more to prevent changes from being made accidentally. It doesn't happen all the time. The file is probably accessed 100+ times a week and it usually only errors out like this about once a week.