There is usually a higher chlorine demand, even without sunlight, when the temperature is higher. Higher temps make all reactions of chlorine go faster, including chlorine reacting with organics in the pool, with pool covers, with pool surfaces and equipment, and with chlorine outgassing to the air. And yes, if there is more sunlight on the pool that that increases chlorine demand as well. With a higher chlorine demand and not changing the SWCG output (% on-time), the FC level will drop.