I wasn't sure what category to put this under, so I'm putting it here under Chemistry.
I noticed that this summer my pool is using more chlorine than in the past -- closer to 1.0 ppm FC per day instead of only 0.5 - 0.7 ppm FC per day in years past. I have an opaque electric pool safety cover so the pool only gets exposed to the sun's UV rays when in use which accounts for the normally low chlorine usage. I keep around 3-6 ppm FC at 25-30 ppm CYA [EDIT] now 40 ppm CYA [END-EDIT] only adding chlorine twice a week (hence the wide FC range).
This past week, there was even more usage of around 1.3 ppm FC per day so I asked my wife about it and she said that she had a friend over with her young daughter so I figured the extra bather load plus possible peeing from the young child might be a reason. My wife has also been using the pool more frequently than in the past -- almost every day for 45 minutes to an hour-and-a-half for physical therapy (exercises) so that also accounts for some higher chlorine demand. When I told her about my thoughts about the young child peeing in the pool, she disclosed that she too would sometimes pee in the pool if the set was long (1-1/2 hours) and she really had to go.
Well at least I now know why there's more chlorine demand, though I'm not sure how close I'll be swimming near my wife on the weekends. So that's one more item to add to the list of possible explanations for unusually high chlorine demand: hot weather, strong sunlight, low FC level relative to CYA so impending algae bloom, high bather load, and wife peeing in the pool. At least pee (urine) is "relatively" sanitary compared to, well, you know!