Is it possible to have final gravity increase in the secondary? I brewed an alt with German Ale yeast and have a final gravity of 1.009 while in the primary. After letting it sit in the secondary for 10 days, the final gravity was 1.015. Is this possible or did I likely misread my hydrometer after the primary?
I think its highly likely you misread the hydrometer. I can’t think of how the gravity would increase without adding sugars. Technically, if it were colder and you didn’t correct for temp it would go up, but 6 points would require it to be MUCH colder I think.
+1. Misread or different temps or both!
You can’t even get there since the maximum density of water is at 4°C.
Maybe your airlock wasn’t on tight and you evaporated 40 percent of your beer off. 
If you didn’t de-gas your secondary sample, you might have had CO2 bubbles lifting your hydrometer.
All right, I’m going with misread hydrometer because it was the same temp and 40% of the beer didn’t disappear from the carboy. Thanks
+1 bubbles, measure again and spin the hydrometer
+1
I once had a similar problem but discovered that I was sliding the hydrometer back into the plastic case too quickly and the paper inside was moving giving me off readings. Broke it and bought another one, problem solved.