Perhaps lack of trust developed when he wasn't honest from the very beginning that he was starting up these friendships?

People grow and change. Perhaps you want the doors closed on your marriage. And starting up new flirty, quasi-flirty, 'lead to'-flirty friends that he met while vacationing with You, that's present day insecurity, with good reason. One of your relationship needs may have adjusted (fidelity) and now that's feeling unmet.

Don't keep torturing yourself by going into his account. Have a serious futures talk. Redefining your needs.