Picture is worth a thousand words, so I'll skip right to it. Here's a closeup view:
Wheel is about 3 months old, has under 500 miles on it.
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Take a look at the specs for the rim (or tell us what kind of rim you've got). There are two ways to join a rim. One is to weld the joint, the other is to use a sleeve inside of the rim extrusion that aligns the ends.
If your rim is welded, then I would be inclined to agree with Carel. But it if is sleeved then it seems likely that what you're seeing is a slight imperfection in the alignment of the ends
There is no need to "adhere" the ends of the rim to each other. The load created by the spokes put the rim in compression. There is no tendency on a built wheel for the ends of the rim to pull apart. The point of welding or sleeving the joint is to keep the ends in alignment.
It is. The rim joints are never perfect after pinning or welding. Rims that are intended for rim brakes are machined afterwards to have a smooth surface. This rim has instead been painted.
The crack comes from the pinned joint. The spokes are pulling the rim together with enough force to keep it from breaking apart.
This being said, rims intended for rim brakes are also thicker at braking surface to compensate for wear. If these are Deep V's, you are fine. If something else, check if there is a rim brake version of the same rim. Rims not meant for rim brakes do not necessarily last long.
I'd say no! There is a visible fissure. The joint was not perfectly welded and opened. I'd take that one back to the shop and have a serious discussion. Of course it could have cracked when you rode over an obstacle and if you hit it right at the weld.