I finally took the time to track down which plant was the cause of the smell and discovered there was lots of it, especially up towards the top. In the picture below you can see it growing in front of the rock bench, looking pretty much like mowed grass from a distance. It's been cropped with a string trimmer a few times. It's in the mint family and is not toxic as the name might otherwise suggest (it's also called American False Pennyroyal).
Bonus today! The aromatic groundcover I figured was some non-native herb/weed planted by the Eklunds is actually native, and it's functioning nicely as a "lawn." All year I have wondered what was giving off such an aroma while we worked on the rock garden.