I'd interpret it as a warning: look behind you, that's where the danger might come from. Used figuratively, you're warning someone that problems or attacks may come from where you don't expect them. Or that someone might be waiting for you to turn your back, so you don't see them strike. You can also use this to describe someone who is acting as though they know they're being watched. "She's been looking over her shoulder all night." (Even if she's not literally doing this.)

I think it just means that somebody want you to pay attention to him,and you didn't know it,or you didn't realized it,he just says this phrase to let you do as he like.Above all is my own special opinion,just for you.I think maybe it isn't right.