Forget about his phone for a minute.
Do you have a smartphone ?
If so, go on a findmyphone website. Samsung do one. Mangobird is a good one as well and is really accurate. Set your phone up to their site. They will ask for your email and mobile number. They're fairly easy to set up. I'm a techno phobe and I managed.
Or maybe you have an iphone?
Accidentally leave it in his car (in back pocket) or passenger side drawer is a good one (somewhere that doesn't look as if you have hidden it on purpose - more a Oh no, I put it there for the time being and forgot to pick it up if he asks scenario.
Oh yes. And make sure EVERYTHING on the phone is set to silent first, ringtones, system, notifications, vibration (battery alert if you can) - Everything.
Next time you suspect he's going to meet her, you can go on your computer, log in to the findmyphone app and send a message to your phone and it will send you back the exact location.
There's no need for fancy expensive trackers anymore.
Anyway, You're only trying to find your phone, which you accidentally left in his car and you wanted to check it wasn't 'lost'. Nobody can argue with that.
Good luck.