Have recently seen this file now in games folder...
_OpenInEditor.lock
It's probably just a temporary file created while a game is being edited. I've tried opening a game in the editor and yes, a file with such name was created, and it simply went away off after I closed the editor.
If you encounter some problems while editing a game, like the editor crashing, for example, it is possible that the file would stay without being deleted as normal. I think it may be safe to delete it, but if it's not causing any troubles I don't see any need to care about it (it will probably be removed normally when you edit the game again and close the editor successfully).
Thanks Iceboty V7000a
The game itself seems to be running ok.
I'll leave it if it still shows.
cheers