I also have those - they mark the edge of the property line, just confirmed that when house was resurveyed and the orange flags were put right next to them.

From the condition of the end of the pipe, it appears it was hammered into the ground.

Let's see. A pipe marker at the corner of your lot... hmm.

Does not appear to be the work of Benjamin Banneker - he worked in stone - so I'm guessing an earlier homeowner memorialized the surveying efforts of a survey team.

Property is surveyed before they build a house on it. That is what they put in so that is legal corner of your property. Good chance the other 3 have been removed as owners somewhere along the way wanted to put a fence up or some get buried over the years. Nice to have it in if a dispute should ever arise with neighbors.(trying to put fence on your property, putting shed over the line or too close to property line as some codes say any building has to be 5 ft into your property or whatever for that city, some nasty disputes arise).

Also, if you check your deed, that pipe might be mentioned as a property line marker.