REV is set to automatically logout after 15 minutes of inactivity on behalf of the user. If a user is working on a document on their screen, their appears as shown in the image below.
If for any reason they move away from their screens/devices for more than 15 minutes, they will automatically be logged out of REV. When they log back in to REV they will see the screen in the image below where the image that the user was working on is no longer active and instead they see a placeholder thumbnail. This happens because your REV login session has been timed out and the access to the image open on your screen gets timed out simultaneously.
This issue can be resolved by just reloading/refreshing the current page. Once the page reload/refresh is complete the previous image will be back on your Rev screen.
How to change the REV timeout default settings?