My spouse has suddenly become very secretive with their phone. Should I immediately suspect the worst?

A real change in behaviour is worth asking about, but ask in order to find out, not to pass judgement straight away without evidence. Allah teaches that when doubtful news comes to you, “then verify it” (Surah al-Hujurat 49:6). Raise your concern openly: ‘I've noticed something has changed lately. Is something bothering you?’ Give room for an explanation before drawing conclusions, because there are many reasons a person goes quiet. Avoid snooping in secret, because if you are wrong, trust is damaged twice. If the concern is well-founded and continues, talk honestly or with the help of a counsellor. Calm inquiry beats burning suspicion. See communication & emotions.