Hi Al,
I do not have a dehumidifier (I did at one time when I lived in a partially submerged lower section of a house in college lol) but your post got me interested.
I took a look around because the first thing that I thought of was cleaning products, but besides Windex I dont think many people use ammonia in their house...
Some other things that popped up are pet waste from thinks like litter boxes, decaying plant material, and just being released from neighboring farmland. The biggest one I would like to know is if these same people that had high ammonia readings had cats...
https://www.dhs.wisconsin.gov/public.../p0/p00810.pdf