I found a YouTube video showing how to remove the rear bumper from a 2019 AW model Polo. In the video, there’s a different shaped rubber grommet / plug with wiring passing through it in the location of your hard circular blanking grommet. I’ve no idea what the wiring’s for in the video or if it’s a factory or an aftermarket installation - it runs down to the bottom right rear corner of the car. You can see it at 8 min 20 sec in the video and again from around 8 min 40 sec to the end of the video.
Link to video;
https://www.youtube.com/watch?v=A4ZLXT_cCk0
As for the vent; I initially thought it was an air pressure equalisation vent to release air from the boot when closing the tailgate so you don’t have to use excessive force when closing it. I’m not sure it is though, as the air pressure equalisation vents on VW’s are normally located behind the rear wheels at the bottom of the rear quarter panel - that’s where they are on the Polo and you can see the left hand vertically oriented rectangular vent from around 8 min into the video, below the rear light cluster and behind the rear wheel.
