I'm running LR5.4 under Mavericks on an iMac with an external monitor. The main screen is the iMac's. I have LR's grid view on the second screen.
Frequently, and for no apparent reason, a floating window appears on the main screen, containing a grid of thumbnails. If I close it, the thumbnail view disappears from the secondary screen also. I can restore it easily enough by hitting shift-G, but it's irritating.
Has anyone else seen this? Is there a solution?