If there are other files/windows open which read data from this file, they may be keeping it open.
What determines if it's in use is the references and dependencies in any other open files/windows rather than the ones in the file you are attempting to close.
For instance, if you are viewing a portal, you cannot complete the closing of the file which contains the portal records. You can close it's windows, but not the file itself, because it must be open while the portal is showing.
For sure it's something like that. I'm not able to see what it is tho. While the file is open (hidden) there aren't related fields on either thelayout I'm on or the hidden file's layout back to file#1. No TOGs in common even. Something, but are there any other possibilities?
Thanks for thinking about it!
Try using the Window menu to see what other files may be open, even in the background. Any of these may be requiring info from the file you want to close.