Cat disappeared?

I've been taking care of a ''used to be'' stray cat for over 7 years. He has been neutered and he's an indoor-outdoor cat. I let him out as usual last Friday around noon (1-25-08) , and haven't seen him since. I could holler for him later in the day and he'd come running to me. I took extra good care of him. He wasn't sick that day,really a healthy cat.Could anyone tell me what could have happened to him? I've looked everywhere,reported him missing and watch for him everyday. He's very territorial and would never have anything to do with anyone. Could he have been sick without me knowing and went off somewhere to die? I've heard animals sometimes do this. Does anyone have a clue what could have happened to him? He;s a 15 lb. cat and weary of anyone. Thank you.