With access to the internet on that ipod, he may be accessing porn sites & doesn't want you to know what he's doing.....or as others said, he may be chatting with other's on sites that he doesn't want you to know about.
Or is could be absolutely NOTHING & he just wants his own space.

However, since he didn't act this way before & it's something that he has just started, I would definitely be very leary & feel that he is hiding something from you that he doesn't want you to know about.

I can understand private journals that we write in for therapy because that is a very personal thing & it doesn't involve anything outside of the marriage or anything that could hurt the marriage.....but when we start hiding things that are links to things outside of the marriage, then you can pretty much be able to bet that there is something going on there that if you knew about could have an effect on the marriage or he wouldn't be trying to be so secretive about it.

Think when our gut feeling waves a red flag, it's something we need to listen to. I know for me, every time I brush off a gut feeling, I regret it in the end.