Check in the battery stats, I bet it says it's charging (green line below the battery stats) even when it's not plugged in. If it does, it's a hardware issue.
My galaxy nexus does the same, I "fix" it by putting a tiny strip of thick paper (for instance a business card) below the "flap" of the microUSB connector (not on the contacts side, the other thinner one). You should see the green bar disappearing, but be very
careful when inserting it!
Some people claims it happens because of different kind of microUSB cables that bend this small flap. See http://forum.xda-developers.com/showthread.php?t=1427539&page=25