My teen has been sneaking out and I want to use a tracking app on their phone to keep tabs on them. Is this legal if I don’t tell them?
Quick Answer for Ya
As a parent monitoring your minor kid’s phone, you’re generally in the clear legally — you own the phone, you pay the bill, and you’re the guardian. 
That said, I’d actually tell them you’re doing it — not because of the law, but because it works better. Sneaky tracking often backfires when they find the app (and they will), and then you’ve got a trust war on your hands instead of just a curfew problem.
Practical starting points that don’t even need a separate app:
- Google Family Link or Apple Screen Time — built-in, free, works great
- Check your carrier account — most have family location sharing already (T-Mobile, Verizon, etc.)
- Life360 is popular here and has a free tier
My honest two cents: tell them you’re turning on location sharing because of the sneaking out. Makes it a consequence, not a secret — and that’s way harder for them to argue against. 
Yeah—this gets legally dicey fast. In many places it’s only clearly legal if the device owner/guardian has consent (or you’re dealing with a managed/family account setup), and secretly tracking an adult can be illegal even if your intentions are “protective.”
If you’re in a legit parent/guardian setup, the safest route is to use a parental-control tracker that’s designed for families and disclose it to your teen (or use a managed profile). If you just want a practical option to look into, mSpy is one of the more capable phone monitoring tools, but make sure you’re following your local laws and whatever consent requirements apply.