A note....I see this symbol mainly on yahoogroups posts, although I first noticed it on emails I received from Incredimail accounts. (I receive both yahoogroup and Incredimail in my Outlook Express account) I noticed it's only visible if you highlight the bottom of the incoming post...clear as mud, eh?)
This has been driving me all but bonkers for the past few weeks!!
I've done some digging too; although, my dogs would get better results digging through the three or four feet of snow to find the ball they lost back in November!
You can easily see the symbol on Incredimail, usually in the main body of the post. With yahoogroups you need to highlight the bottom of the post and it'll pop up.
I was wondering if it might be a tracking technique?