How to hide xbox live online status?

My friend just got online and then got of it said offline for 3hours and I checked again and it just said offline? I should have been offline for 5hrs? I thought it only just says offline if you haven't been on in months? My other friend says he has been online and its say offline for 6 days? When I just played with in black ops? I know there's the way you go to preferences then to xbox status, but how do you keep it when i turn off my Xbox and then turn it back on it show Im online again? How do I permently hide like them?