I installed a wheelset from my collection last night, which I had not ridden for a few months. The wheels had been used in predominantly dry conditions for ~ 300 miles back in the fall, when they were new, then removed from the bike and stored a hook in my garage over the winter.
Today I was getting the tire pressure set for my ride to work, and noticed that the front wheel wouldn’t spin. I centered the brake caliper, and re-greased the thru axle. Still, the wheel produced a low hissing sound and would stop spinning after a few revolutions. Since I needed to get to work, I decided to just ride it. I did not remove the end caps to inspect anything, due to time reasons
About 1/2 way through the ride, I go up a short hill, and at low speed, I could really hear the hissing sound, so I figure ok, one of the bearing is just shot. But by the time I got to work, after 1 hour of riding, the wheel spins freely without noise.
The hub is a very standard sealed bearing DT-Swiss inspired design from Light Bicycle.
Has anyone had this happen before or have any ideas what could have caused this? I am not experienced with servicing hub bearings, so it is literally impossible to condescend to me 
The hissing sound could be a number of things, but all lead back to one or both bearings needing to be replaced. My money is on the axle spinning within the bearing inner race, the bearing being seized due to corrosion. Once the corrosion breaks free, the bearing can spin with relatively low effort, but there will be roughness. If there is still moisture present inside the bearing, it can seize again when left stationary for an extended period of time.
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Sounds like one that should be easily diagnosed by pulling the endcaps off when you have a chance. Either there was a piece of dirt that got trapped and then evacuated or ground away or the above mentioned corrosion. This could even be the endcap having a little dirt in it or being slightly crooked and not the bearings. 300 miles shouldn’t be anywhere enough to have wear-related problems but even a small amount of surface rust over time could cause a problem.
I could be wrong, but in the event that you’re unfamiliar with disassembling your DT-Swiss-esque hubs, you simply need to pull off those end caps. If they’re a little stiff and hard to remove, I’d use a bench vise (very gently, and with clean rubber pads for the vise jaws) on the end cap to aid removal.