Addons install in their own docker container. The “/data” folder would exist within that container, but would also be mapped to an actual folder on the host. As far as I can tell, all of the data for addons live under:
/mnt/data/supervisor/addons/data
If you log onto the pi directly (not using one of the terminal/SSH addons) and enter the following:
ls /mnt/data/supervisor/addons/data
You should see folders for all the addons you have, and (hopefully) one will look like SunGrow (with other numbers & characters at the front). That folder will (normally) correspond to the “data” folder in the container. Use “ls” to then look inside that for the file. Note that the dot at the start of “.GoSungrow” means it’s a hidden folder, so “ls -a” will show it, but “ls” by itself will not. To display the content of the file, use “cat”.
(PS You can use terminal/SSH if you’ve opened port 22222, but assuming your pi has a keyboard and display connected, that it much easier. You just can’t access with any of the port 22 SSH addons. There is a “temporary” addon to configure port 22222 more easily should you need it)