HI,
This Thread at the 8th May 2012 spot
It is not clear what causes it.
It does seem to be linked to occasions when the DOCK count starts at a + figure even when there are no new ones and then counts upwards until messages are Read but does not drop below the starting figure.
Computer restarts or PRAM resets seem to be the simplest "cures".
The Caches do seem to have a database of everyone you have chatted to (Although I have System Preferences > General Pane set to 20 - (this used to be for Documents separate from the others) - which in turn is the Number of item in the Recent Items list in Apps - so mine might have those listed as Video and Audio chats are listed separately from Text Chats.
com.apple.imagent.plist holds the info about whether your return a "read" receipt and might be a more likely candidate for storing the DOCK/Icon count.
8:55 PM Saturday; June 1, 2013
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously