It sounds a bit funny to me personally. I think the closest would be "expect me" like a one-liner in an action movie or a threat; meaning that you'll appear out of no-where.

It sounds fine to me! Perhaps it's a regionalism... It means 'I can't tell you exactly when I'm going to arrive'. For example, you might tell your parents that you're coming home to visit them for the weekend. You're going to set off after you've finished work tomorrow, but you don't know precisely what time you'll leave - it depends on how busy you are at work. The time your journey takes will depend on the time that you leave and also on how busy the traffic is - if it's rush hour you could be delayed, or you could be lucky and miss the rush hour. In other words, you can't predict when you'll arrive. So you say to your parents 'Expect me when you see me'. The inference here is just as you say - don't wait for me especially. I'm going to arrive, but I can't tell you when. In the UK, at least, this is a normal thing to say.

I've heard the expression a lot and have used it myself , many times. Usually it's an informal way of saying you are coming but you're not sure the exact time. For instance, I've been invited to someone's house for dinner but I've been delayed. I might call them and say "Expect me when you see me". It means I will be coming but I'm not sure exactly when. Or perhaps I am making plans with someone to meet them somewhere. I'm not sure exactly sure of the time when I can meet them because other things are happening, but I do intend meeting them. I would say, "I'll see you at the restaurant, but I'm not sure what time exactly. Expect me when you see me." Keep in mind It also can be said sarcastically to mean the opposite - you have no intentions of seeing them - and so you say this in a sarcastic tone or in a joking sort of way. So be careful in the tone you use :)

It does not mean anything actually.