They are typical as low-point drains and manifold drains, both on the main line (always charged). Don’t just cap it if that would make a dead end, especially if it’s sloped down to drain. I can’t tell very well from the picture, but if it’s on a tee you would be okay putting in a threaded cap.
A thin layer of glue stick kept petg from bonding to glass for me, but a nice thick layer of G10 is better as long as you heat soak it before printing. I roughed mine up with some sandpaper and haven’t had any issues. Just remember good ppe when working with fiberglass.