When you monitor a power line, you don't get a tcpdump showing who's producing and who's consuming the power. You can't just jump straight to the culprit.

It's just bulk flows, so when a weird flow shows up, you've got to move up or down the line, past a few producers or consumers, and see if the flow changes. For a subtle effect, that might not even be so clear.

Worse, for an effect that's most visible at the medium-voltage transmission level, simply placing a monitor might take weeks. These are planned installations with incredible safety considerations -- you don't just plug in.

Finally they found a research campus with a bunch of battery storage systems, AND the ability to install a bunch of monitors, and persuaded them to install dozens of monitoring points. Finally a high-resolution view of the effect allowed them to confirm the behavior.