Meaning of CrystalSVG folder-with-plug icon ?

I'm running Slavek's very excellent TDE 14.0.1-pre on Debian Jessie, and I use the TDE CrystalSVG icon set. I recently noticed an unusual icon on one of my folders. It could have been like this for a while as I don't often use this folder, but can anyone tell me what the "plug" icon in the following screenshot signifies, please ? http://www.leverton.org/temp/TDE%20folder-with-plug%20icon.png I have checked in Properties that it hasn't been manually set to this. It hasn't, and I can't see a "plug" icon or overlay anyway. The folder has the same filesystem attributes, extended attributes and user attributes as its siblings, so I think it must be some property only known to TDE that's causing the plug overlay to be added. It's no problem to me, but I'm curious to know what property that might be. Nick -- "The Internet, a sort of ersatz counterfeit of real life" -- Janet Street-Porter, BBC2, 19th March 1996