Unknown hole in head of B5254T on Volvo C70 2.5T

[reise]

Hello.

Under the engine spark plug cover, there is a large hole with two small threaded holes close to it.
When I bought my C70 the hole was empty and dry.
I show this hole in the next photo.
The photo dates back to 2012.



In 2014 the water cooler started to have a small liquid leak. I should have immediately replaced the radiator, but I used it for another year and a half. Then the water loss from the radiator was increased.
So I bought a new radiator and with the occasion I had the head gasket, distribution kit, other work replaced. However from that moment engine oil started to come out of that hole that was once dry.
I show this hole in the next photo.
The photo dates back to 2016.





The mechanic did not want to take responsibility for the damage.
And instead he "solved" the problem by plugging this hole, to my great disappointment. I haven't had engine oil leaks since then, and the car works great, but I don't know if this was the right solution.
What is this hole for?
I show this hole in the next photo.
The photo dates back to 2017.