Grooves in new brake rotor?

How deep are the grooves? Sometimes a small pebble finds its way in there and can cause small grooves to form. Also, if you think about it, a part of a standard brake job is to machine the rotors if they're being reused, to remove the grooves and surface unevenness. The linings on the pads will wear to match the grooves, but since we're starting with new, flat pads, we want the rotors to match that.

Be aware too, in the past it was common to get brand new rotors with a groove on each side, right in the middle of the friction surface. Many people thought those were to form a ridge on the pads to keep them in place. In fact, their purpose was as wear indicators. If any of each groove was still there when starting a brake job, the rotors were thicker than the published minimum thickness, and could legally be machined. There's two thicknesses specified. One is the "machine to" spec. That's the thinnest a rotor can be machined to when truing it up. The second spec was the "discard" spec. That was commonly 0.030" thinner and was what the rotor was allowed to wear to after machining. Today most rotors are made really thin to save weight, so they're usually below the discard spec by the first brake job. The good news is they cost just a fraction of what they did back in the '70s and '80s.

That sounds like something you'd never be aware of if you weren't looking there. You'll find those on almost every other car you look at. I don't think you have anything to worry about.

I should mention one more comment of value. A lot of new rotors are made in China. There's absolutely nothing wrong with their quality, however, when we make parts out of cast iron, we set them aside for 90 days to "age" before they get their final machining. The Chinese cast 'em, machine 'em, pack 'em, and ship 'em, then they age on your car. It is very common for them to warp in a few months resulting in a brake pedal vibration. Return them to the store for a light machining to true them up, then they'll be fine for the rest of their lives. If you demand new ones under warranty, you'll just have the same problem in a few months. Most auto parts stores have a brake lathe so they can do this for you at no charge. At worst you'll have to take them to a repair shop that has a brake lathe.

No different than finding mud splashed up on only one side of the car, or a bug splattered on one side of the windshield.

If there was something major wrong causing the groove, you'd be hearing it as a grinding sound, and the groove would get worse over time. If you're still worried, post some photos of it from a couple of angles.

Dandy photos. There's nothing unusual there. In fact, in the second photo you can see some remaining sideways scratches that were put in with sandpaper to develop a non-directional finish. That breaks the grooves down that get cut in by the cutting bit on the brake lathe. Machined rotors end up looking like the grooves in a record album. We always try to take a very light final cut to minimize the depth of those grooves. They will wear down smooth over time, but if we leave them too deep, it can cause a pad to act like it's running on a screw thread. It will walk as far as it can, then snap back and start all over again. You never hear that because we don't let a rotor leave with that deep of a cut.