Exes and phone pins

My fiance is 14 years older than me and his ex was quite a bit older than him, there's probably around 25 years difference between myself and her but I often look at him and worry that he'd go back because she was motherly to him. When they were together, he never said a bad word about her, everything was positive but whenever I saw them together he looked uncomfortable, like he was unsure if he was allowed to speak to anyone else etc, I'm just soo happy with him that I don't want to lose him! He also has a fingerprint pin on his phone and spends quite a bit of time on it, usually playing games but the other day he picked his phone up and asked if I'd been on it cause it was asking for a backup pin. He sounded startled so I'm now wondering if there's anything he's hiding on there. Am I just being paranoid???