Caught In The Act of Spying on My Teenagers' Texts

Alright, so here's the deal. I've been sneaking a peek at my teenagers' texts lately whenever they forget their phone in another room or drop it somewhere where it locks up and I can swipe through. Yeah, yeah, I know it sounds like I'm invading their privacy and all that, but hey, when your kids are 16 and 17 and suddenly get super secretive about who they're talking to on the internet and how late they stay awake at night messaging, you start wondering if there's a reason. The funny thing is, nothing bad has really come up yet, just normal teenage stuff like worrying about homework or what movie they'll go see with friends this weekend. But I figure it's my job to be ready for anything, right? So here's the real confession: I've been keeping notes on their contacts and texts to stay one step ahead of any trouble. Am I totally paranoid over nothing? Or is this a normal parent thing?