My spouse always turns the phone upside down so it won't be visible

Recently, my spouse’s behavior has been making me uneasy. They never take their phone out of hand, but whenever I sit next to them, they flip the device over and put it on the table. They didn’t used to do things like this; they were more open before. When I ask about it, they brush it off by saying, “You’re just being unnecessarily nosy.” Honestly, I don’t feel comfortable at all, but I also don’t want to be unfair. What do you think should be done when you notice such behavior?