There is so much information we don't have. For example, why does he feel the need to spy, what has happened previously, why is the phone locked if there is nothing to hide on it. Is he actually spying, or trying to keep track of usage and control the budget. I used to keep track of how many text messages and minutes were used on my phones, before we were able to get unlimited usage, and I would question my wife about her usage if she went over.
If I had to lock my phone to have privacy from my spouse, it would be time for divorce. It would also make her think I don't trust her, and have something to hide when I don't.
Trust is is a primary requirement for a relationship.
If I went to use my wifes phone, and found it locked, I would think she is trying to hide something from me, even if she wasn't, why else would she lock it without letting me know about it.
Anything from the past would reinforce any new distrust as well. Example; my wife cheated on me 10 years ago. No matter how hard I try, I cannot overcome that breech of trust. It will always play a role in our lives now.