"http://captive.apple.com/hotspot-detect.html" ?????

If that Mac was within wireless range of a "captive portal" and you connected to that network at some time in the past, then you will see that page — the one containing the single word "Success" — upon successfully reconnecting to that network.

That is all it means. There is no reason to consider its appearance to be related to any nefarious activity whatsoever, other than the possibility someone could have physically transported your Mac to a location within range of that captive portal, and then physically transported it back to where you found it, all without your knowledge.

You might consider changing your Mac's network preferences to prevent automatically reconnecting to that wireless network, to require your Administrator password when changing wireless networks, or both. Those options can be found in System Preferences > Network.

It means that you, or someone else using your computer, connected to a wireless network with a captive portal.

Is there a way for me to know or to find who could be connected on my wireless?

If you mean your wireless network, that's not relevant. The computer connected to a different network. You can see which network you're connected to in the Wi-Fi menu.

I've been experiencing some not-normal behavior with my MBP (if you know what i mean..)

No, I don't know what you mean. You would have to tell me, or perhaps better, start another discussion with a more descriptive title. I can't explain why you saw an indication of connecting to a captive portal. I can only tell you that's what it was.