unusual to have a manhole on private property
More common than you think.
We had a huge drainage pipe buried under our property. I vividly remember the foreman of the work crew knocking on the door to tell us to stay inside until we get the "all clear." So, i went to the kitchen window and watched as the whole backyard did the "wave" when they set off explosives underground.
The pipeline ran under our down-sloped driveway, under the car parking area, and all continued past the back of our property.
There was a creek next to our house that normally became a river during heavy rains. The pipe followed that creak on our side and carried the water that ran into the street's storm drains. It also became the sewer system, which we were able to connect to and do away with our septic tank.
My grandmother was paid for the right-of-way, and the only evidence of the buried pipe was a concrete manhole in the back of our property about a foot or two above ground level. Similar to this:
When my mother sold that house, the city made her an offer that was almost 50% more than the best price she could have gotten on the market. The house is gone now. It's just a vacant plot of land next to the creek.