Ahh...it's where what looks like a bung, to the right of the label is located. There was not a bung fitted on mine.
That said, there were no visible signs of water or dirt ingress inside the unit when I took the cover off, everything looked as clean as a whistle.
Cheers , haven’t been able to see what that’s for, wonder if u can get a blanking grommet for it just in case to. prevent any ingress of water etc
If I find anything will post up a link for u