Persistent musty snell inside this kitchen cabinet

[adamt11]

I've cleaned the cabinet thoroughly but the musty smell always returns. The countertop above is dry. None of the other kitchen cabinets have this issue.
I've noticed the small hole in the top left-hand corner. I wonder if that could be where the smell is coming from.

Drawer:

 

[just pumps]

Can you remove the kickboard/ plinth and see under it.

 

[chirpychippy]

Take the plinth off and if you have a mobile phone with a camera take a picture of the void behind the units, better still take a video of it,see what that shows....

 

[Koolpc]

Probably mould behind there...