Visited on February 27 without charging.
The mysterious second charger is in the underground parking, quite close to the entrance.
I'm not sure where you enter/leave on foot. You can of course walk through the ticket gate like I did, but the official way seems to be through the pool building.
This charger 174749 is listed as 11kW in Swisscharge app, but the sign behind it says 22kW, like outside.