The body doesn't store protein in any real capacity. Fat is stored in adipose tissue and carbs in glycogen, but protein must be available in the bloodstream when needed for muscle maintenance and synthesis.

Even if you up your dietary protein during the feeding window, if you're fasting for lengthy periods of time where the protein has already been utilized for fuel, then the muscles are starved and catabolic. There's no passive protein depot built into our physiology muscles can pull from.

Is it possible some of the area it uses protein from is old cells? I'm thinking parts of digestive system, immune system, old cells it scrubs and reclaims some protein from when fasting. Still catabolic but useful catabolic?

It is called autophagy and happens regularly in our body, but in very small amount. Do experiece greater results, you need prolonged fasting (3-5 days)