When InDesign crashes or is force quit it would leave behind the lock file, and there are reports of some network situations that fail to remove the lock files when the document is closed.
I've experienced that as well in the past (idlk file created after InDesign crashes), but this time the file did not crash the last time it was open. Might there be another explanation?
Is it stored on the network? Are you sure the last person to use it closed it properly?