My cat is periodically off colour for no apparent reason why?

My cat came home last night after doing what he does??? He was fine last night but today he seems dull and tired. Routine is the same every day but he has been off his food for a few days, but so is my other cat(off his food but not unwell) My other cat is a house cat and not ready to deal with the outside yet i don't feel. They are both fully innoculated against everything, wormed and de-flead. Why is he going off food and appearing dull. He is 11 months old. His gums are fine, weight, and coat fine. I feel that he maybe having too much food choice? As i say we have an indoor cat that has gone of food too. We are buying reputable brands of food and supplying dried food that is available at all times for them both. My unwell cat has his freedom but seems to be suffering from depression or lethargy today. Is this normal behaviour in cats i have never owned a cat before and not sure if this is a regular occurence with experienced cat owners.