Unless he owns a lot of acreage, I'd think of it as in the area. Ohio is big enough and the map small enough, that each triagle covers a couple of acres or so. Also, he probably spread them out a bit instead of overlapping triangles everywhere.
As I understand it, Mills didn't visit all the mounds himself, he used basically everything that Moorehead and Squire/Davis did before him, and then added a lot to it himself and from local resources. Generally speaking, if Mills says there is a mound there, there is a mound there but it might only be a couple of feet high by now if it's been in a corn field for the last 90 years... Also, his work served as a road map for many, many generations of Ohio graduate students, local historical societies as well as many early collectors, so the mound very well could have been dug up by now.
Try to locate the stream near it and hike around, it should be easier to see now that the leaves are down.
Here is a low res picture of the map, you can get a very hi res picture online.
he has tons of acresand it shows the triangle all in his acres but not in his fields but his tree line. im going hunting for it tomorrow. it looks like it could be in his washed out like creek