Same problem here. Some stations are not creating a lock file at all, allowing for a read-only copy to be opened via double-clicking... OR ...if this is possible, the lock file is being created in the wrong folder? Saying this because at times an .indd file within a folder where there are no lock files showing will open a read-only copy of the file upon double-click. I checked this with another user on our network: She had a file open and we both witnessed no lock file in the directory of the file she had open. I navigated to the same directory and double-clicked on the file opening a read-only file. She was then unable to save that document until I had closed the read-only one.
I hope this information can lead to a solution.
You replied to a months old thread with no details to confirm what you are having happen is the same issue. I've therefore branched it to its own discussion.
Now, if you can give us information on your InDesign version, operating system and network, we might be able to take stab at it.