Went round and round today with the realtor and SE guys. Curious what everyone else thinks. 
First thought was main water shut off valve at the bottom of the pit.
How big around and deep was the hole?
Stop and waste valve was located about 3 feet away.
About 10" dia, and at least 2 feet deep, I poked around through the debris, but I didnt want to be the one cleaning it out 
Same thought as Stephen.
That’s where you throw the Ultra beer cans. Just looks like someone missed is all 
I see you are in Big Bear. What about a covered trash container to keep bears or raccoons out?
what about a form to pour concrete in for a post?
My first thought but why the wood frame? Is the pipe loose?
It seems like form tubes would be a heck of a lot cheaper: 
http://www.thetoolworkshop.com/quikrete12concreteformtube692203.aspx
Thanks for all the input.
Hmm… No reason I can think of for a post or footing there. I just went with “possible water main shut-off” which was my original guess, it just seem very large for that purpose and like I said there was a fairly new stop and waste valve near by, so maybe they just installed a new one and left that in place? Idk, told them they should mark it though, someone would be in for quite a surprise if it was covered by snow and didnt know it was there.
I am an X smoker and I recognizance hiding your Butt kit from the wife