AITA for thinking my BF is cheating because of his locked phone?

Title: AITA for thinking my BF is cheating because of his locked phone?

I (46F) have been dating my boyfriend (55M) for 9 years. Here is my problem: I have already caught him cheating two times in the past.

Now, I strongly suspect he is doing it again because he keeps his phone completely locked and refuses to give me the password. His excuses are, "You don't need it because it's my phone, and I don't get calls or texts on it anyway," and "It's the only thing I have that is just mine." (For context, he also has a car and a laptop that I am not allowed to use).

Given his past history and this sudden secrecy, is it reasonable to think he is still cheating? Am I the asshole for suspecting him