Yes, it would appear this mysterious recurring poison ivy is indeed possible. I had a horrendous case on my right hip last year (long story). Had to use steroid cream, and went to the doctor 3x because it was so bad.
Now a year later, I had a very minor P.I exposure on my left ankle. A week later, that same spot on my right hip flaired up again. The hip was not re-exposed! I was not wearing any clothes or boots from last year, as several are suggesting... it is the exact same spot recurring. My body's histamine reaction to the ankle must have made the hip area flair up again.
I'd like to avoid steroids this time around, so I'm going to try acupuncture.
Oh, and I am highly allergic to mango skin. Good call on that one.