Homebrew gives a warning: "You have MacPorts or Fink installed"
Note that some 3rd-party packaging tools may sometimes automatically install MacPorts. I recall RVM does that (or at least used to, I don't know the current state), so while you're not aware of actually installing it manually, it might still be there.

If you move /opt/local to a different location that might break whatever pulled in MacPorts in the first place.