So what does this mean?
Do they think the Palermo tunnel might be leaking?
No. The Palermo tunnel is lower than the wet area by around 100ft. The slope of the tunnel in the bedrock is not much - so the tunnel wouldn't be a source for water seepage 100ft higher.
The test drill hole will have a piezometer sensor for analyzing for water presence & pressure. This is to monitor for seepage in the hillside next to the dam embankment. I too was suspecting a possible water table percolation from somewhere on the hillside as the wet area greening doesn't always follow/appear based on reservoir levels. There seems to be another component in the mix (spring?, water percolation delay from pore pressure compaction de-compaction from reservoir levels?, hillside pooling?,)