Support Mystery icon in Notification Bar

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http://androidforums.com – I searched the forums and couldn't find anything close to the mystery icon that showed up recently on my HTC evo 3D: It's the second icon from the left, between the More Notifications icon and the Data icon. I've been trying for hours to figure out what it is. I'm sure it's a custom icon from some app, but I haven't installed a new app recently. Naturally, it doesn't show up as a notification when I pull the window down. Anybody recognize it? (General)