It must be the same, although the notion of "my back" means it's also not in front of me or to the side, so perhaps the speaker feels even more claustrophobic and creepy than the other expression would signify.
I think if a person is behind another person's back, then how can he actually watch closely?! You can peek if you are behind someone's neck... But you are behind back, neither him nor the person who is being watched can actually watch...
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