Blue square with count of unread mails

This blue square with a white number in it, quite large, has suddenly 
taken to appearing on my desktop. The number shows how many unread mails 
I have in whichever mailbox I'm looking at. I've never seen this before. 
It doesn't harm but I wondered why it suddenly appeared.

I've been running 6.2.4 for about a year and 10.4.11 since it came out.