Why does he always clicks off his phone when I walk into the bedroom?

Both my husband and I work from home. He works in our bedroom at our desk and I work in the kitchen at the table. It seems that when ever I walk into the bedroom he's playing around on his phone but when he hears me walking up he's quickly close out of whatever he's doing. He feels that I'm only coming into the bedroom and only looking at his phone just to try and catch him doing something. He likes to play games on his phone but there are times when I'll see that he's texting someone. He says that he has friends that I don't know and he shouldn't have to announce ever text message or phone call he gets. Today I walked into the bedroom tog et our granddaughter who was sitting on the bed and when I got to the door I just stood there. When he noticed me there he closed out of whatever he was doing. He feels that if I was just coming to get the baby why would I get to the door and stop? He said I should have jut walked in and looked straight at the baby and not at his phone. He says I'm trying to catch him doing something. He even has his personal laptop sitting on a tray next to him and he'll turn it slightly so I can't see what's on his screen or if he's typing something I'll walk by the bedroom and he immediately stops typing. What's he hiding from me? He feels that even in a marriage everyone is entitled to some measure of privacy but I find it suspect that whenever I go into the bedroom I always catch him on his phone between calls for his job.