I used to sneak out too when I was about that age and also hang out with much older friends - however I never did it on a weeknight. If you know how he is getting in and out I would do something that alerts you when it is happening or make sure it isn't an access eg. a window lock with a key or as PP said make sure it makes a loud noise. You could also hide a baby monitor in his room so that you can hear him if he does get up

However, I would probably only do this if discussing things with him upfront doesn't resolve anything. Sit him down and explain that you know he wasn't in his room last night and you know that he was on the phone etc and hopefully he will tell the truth.

It's tough - I'm not really looking forward to the teenage years (especially as I know what I was like!!)