Engine bay hole

Hi

i bought a VW polo 6n2 year 2000 almost a year and half ago. While doing A/C repairs and other clean ups, I discovered the reason there is an unbearable heat coming from fuse box side under the steering while was a whole behind it. I never saw it before but that hole on the other side under the coolant reservoir next to the break fluid container. 
 

what is it? It does not seem as a intentional cut, but i cannot find any reference for it to find like a plug or something. 
 

how can i permanently close it?