Could be a small slave leak, soaking the shoes without reaching the outside of the drums yet. A sure sign would be a slightly lower master level.
I would start with a visual on all four corners and master fluid level. Check lines, look at backing plates, etc. Look for small puddles under the vehicle. Eyeball the tires while you're at it. Look for separating tires, low air, etc. Eyeball your suspension while you're at it. If everything looks good, adjust your brakes and go for a test drive. If it still pulls, break out the jack and jack stands and start pulling drums. Start with the front axle as that is the usual area. Issues with the rear brakes can cause pulling, so pull those drums next. Keep looking until you find the issue, then repair it.
