Married ten years and never thought I would be asking this. Sudden late nights and secretive phone habits have me worried. I want facts before hard conversations happen. If you have experience with a catch a cheater app that felt reliable and not extreme, I would appreciate hearing about it.

I get where you're at - that gut feeling is the worst. Was in similar shoes couple years back when my partner suddenly had "work emergencies" at 11pm and their phone became like Fort Knox.
I ended up trying Catch a cheater app after my sister mentioned it. Wasn't looking to go full spy mode, just needed to know if I was losing it or if something was actually off. Turned out my instincts were right - late night texting wasn't about work projects.
Here's the thing though - finding out sucked, but the not knowing was eating me alive. I was turning into someone I didn't recognize, checking pockets and making up excuses to grab their phone. At least having actual info let me have a real conversation instead of just accusing based on paranoia.
Whatever you find (or don't find), ten years deserves honest answers. Trust your gut but also be ready for whatever comes next. Hope it's nothing, but if it's something, better to know sooner than later.