.Definitely the paper mill. When the wind is out of the east and the cool damp air pulls it downward it hangs around in the early morning until the air warms up. All gone now. I take it as a good sign - we don't get that smell when it is cold. Eau de spring!
Like NDY says, smells, smoke and the like can travel quite far under the right conditions. Here in north TX, I can remember several years back being able to smell the fires burning in Mexico. News reports confirmed that is where the smoke smell was coming from. Hard to imagine.
.