Filling a mystery vent.

[dcoy]

Hi
We have a mold problem in a kitchen cupboard. The cupboard is suspended, in the corner of the room against an outside wall. I was about to install a little ventilation system to get some airflow thru but then remembered there’s a vent on the outside leading straight to the back of the cupboard.
It’s about 15cm deep (measuring from the outside) and tapping the back makes a metallic sound from behind the cupboard.
Appears whoever installed these cupboards put them straight over the top of the vent.
So im now considering cutting out the Masonite at the back of the cupboard to see what’s going on.
The thing I’m not sure about that is appreciate an opinion on is as this vent isn’t doing anything but potentially drawing condensation to it, should I fill it in? And if so what with? I don’t want to make a huge building job out of it, but at the same time I’m not about to start blasting expanding foam in there.
Thanks for any suggestions

We get some mold issues

 

[Harry Bloomfield]

It's probably an air vent, installed for an open fire, or range. If there is no longer and open fire or range demanding an air supply, you can safely brick up both the outside, and the inside of the vent.