Is there some sort of plug missing here?

[krusing]

Adding a catch can is one thing that can be fully reversible. When it’s time to sell, if your concerned that a catch can sends the wrong signal to prospective buyers, just return it to standard an nobody would be the wiser and then you can use the bits on your next car
Don't use a pressure cleaner within a modern engine bay, just too much potential for drama. Use a degreaser spray can (not the really strong blasting ones) and then a soft water spray to rinse it off. Soft water spray similar to a slow shower head one uses for delicate flowers

It’s still a good idea to avoid the brake master cylinder reservoir and alternator (temporarily cover them with a plastic bag). Then you should be safe while getting things nice and clean

PS: if things around the sump are covered in thick greasy dirt, use a small spatula and scrape as much off as you can before using degreaser. The thick greasy crap can be disposed of at the local oil disposal sites that councils run. In this way we can reduces the crap that invariably ends up in the storm water system and thus out lakes and beaches.

After spraying on the degreaser, I sometimes use one of these [Parts Cleaning Brushs]
Ideal for hard to get at places.

https://www.supercheapauto.com.au/p/toolpro-toolpro-parts-cleaning-brush---plastic-handle/12559.html#q=cleaning+brush&lang=en_AU&start=2