OK - You didn't mention if you can gain access in the house (to the basement or crawlspace) that's CLOSEST to where the pipe comes out of the ground. Once there, start looking for pipes that go OUTSIDE, and start tracing it/them BACKWARDS.
You could try 'snaking' the line to see now far the snake goes. Plus, it may give you a clue as to how far 'in' the pipe goes before it makes a 'turn' (assuming there is a turn.)
At worst, consider digging up the area VERY CAREFULLY!!! This may be your only resort. Once you've determined what it goes to, BE SURE TO POST BACK TO US. I for one would LOVE to know what it belongs to.
My best to ya and hope this helps.
Jay J -Moderator