Line tester reads power to 220V outlet when the breaker is off
I'm not clear how you traced the wire, unless it's exposed the whole way?

If you do have access to the wire, even if it's just when it enters the panel, you should use a non-contact voltage detector to check for power. When you flip the breaker, it should go off.

There's a possibility that there are two feeds to the outlet. I would start by going through all breakers one-by-one (or several at a time, to narrow it down) to see if the power shuts off. 

Keep in mind that you can get 220 by bridging two 120V breakers, and so one single breaker may not turn off all power. This setup is not legal, but it sounds like it may not be wired correctly anyway - don't make any assumptions when it comes to bad wiring! 

Worst case, shut off all breakers, and confirm the power goes out. Then start turning things back on (an assistant and/or non-contact detector that beeps will help you here).

I personally would not touch any of those wires while my meter still read voltage, and I recommend no one else does either.