hole through basement floor near drainpipe

I plan to finish part of the basement in an older home, but I've come across something I don't understand that was probably done during a renovation. I need advice before I can proceed.
The renovation involved a sink/dishwasher drainpipe (PVC). It travels down about an inch from the basement wall, and then into the floor. You can see where a trench was made in the concrete floor to hold the pipe. The trench and pipe have been covered with new concrete.
This spring we had a bit of dampness from seepage (an outside drainage issue, I think), and most of it dried up in a week or so. But between the pipe and the wall at the basement floor, the dampness persisted. The concrete there was also lumpy and ugly. Finally I dug it up; it turned out to be a patch of crumbling concrete, very sandy and easily broken.
I excavated down about an inch and a half to the original concrete, which is still in good shape. The rotten stuff is all cleaned out now, and I plan to put in some epoxy cement and then some patching cement. But here's what I don't understand: at the edge of the trench, about two inches from the wall and about six inched from the pipe, on the upslope of the basement floor, there is a hole, about an inch in diameter, right through the foundation to the earth below.
I'm sure this hole has nothing to do with the drain as such, because I ran water in the sink and not a drop of it appeared anywhere in the excavated area. I could hear it going through the pipe, but there was no leak from the pipe.
Why is the hole there, and what should I do about it?
-- "For it is only of the new one grows tired. Of the old one never tires." -- Kierkegaard, _Repetition_
James Owens, Ottawa, Canada
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