Is this a cleanout or drain?

A week ago, a plumber replaced a failing gate valve with a new ball valve on 3/4 inch copper pipe. It's my main water shutoff for the house. The day after the installation and for the following four or five days, I noticed a drop of water would form every several hours on the valve at the "seam" on the body of the valve (see attached picture - look near the center of the picture at the horizontal seam on the valve). The water does not seem to be coming from any of the obvious openings like near where the handle gets connected or where it was soldered. For the past couple days, I have not seen any more water leaking as described above. I was told by a representative of Webstone, the manufacturer of the valve, and someone else who works in the plumbing section of a local hardware store, that the leak likely occurred because of over heating during the soldering. I have a 30-day warranty with the plumber. Should I have the plumber come back and replace it with a new valve? (I'm concerned that, even though it has stopped for now, the leak may show up in the future and maybe get worse.) What do you think?

I live in a ranch/concrete slab style house outside of Chicago, built in the 1950s, but with some renovations since. We have well water, but are tied into the city sewer system if any of that matters. I've noticed a faint smell that has been getting worse, and after removing everything else, I'm convinced it's coming from this PVC pipe/opening in the floor. [IMG]https://image.*******/b0XKJS/IMG_20180302_112952.jpg[/IMG] I'm able to easily unscrew the top (there doesn't appear to be any rubber seal or anything that would make an air-tight fit When I remove the top, I can see that the pipe goes down a bit, and connects to another pipe down below. There is visible water but it looks nice and clean. With it open, I can confirm that it is the source of the smell. [IMG]https://image.*******/bRn8Cn/IMG_20180302_113002.jpg[/IMG] My question is....what do I do to fix this? My first thought is to just make the seal airtight....cover it with something? But I figure there is a 'correct' way to do it and whatever I do won't be correct unless I get some advice from people who know. Can I just caulk the thing shut? Thank you EDIT - also if anyone can tell me why my images are blocked out/not rendering I'll fix em