ok, here are pics..

{{gwi:8532}}
{{gwi:8533}}
{{gwi:8534}}

Is it a capped well?

The bolts seem to indicate something was once attached there: hot tub or gazebo seem most likely.

There's only one pipe, so that seems more likely to be electrical rather than plumbing, and therefore a gazebo rather than a hot tub. Have you tried shining a strong flashlight down the pipe, and if so, what do you see? Anything that might be wiring? The "clog" a foot down is probably simply a bend in the pipe.

Just to rule some things out: Are you on sewer or septic? Are you on municipal water, or do you have your own well?

Did you ask the realtor what it is?

Have you asked the neighbors? (Not simply the neighbors on either side of your property, but people in the neighborhood who might have lived there for 15 years or more, particularly if they were close friends of the people who lived in your house.) If you're adjacent to any of the country club facilities, you might ask them also.

But in the meantime, I vote for the ghost of a long-gone gazebo.