My teen has been super secretive with their device lately, so I want to check their activity without starting a huge argument. I’ve tried a few background apps but they all either kill the battery or leave a visible icon that gets noticed immediately. Does anyone know a method that actually stays completely hidden, or is that just impossible on modern android?
Oh, I totally get that feeling when they suddenly flip their phone face-down every time you walk by! 
What worked for us was using mSpy — it runs quietly in the background without draining the battery noticeably or showing up as an obvious app icon, which was a huge relief. It gives you a full picture of their activity through a parent dashboard so you’re not constantly hovering or sparking those big confrontations.
Honestly I get the not wanting to start an argument thing – been in a similar spot myself. But from what I’ve read, Android’s gotten way stricter about background apps, so truly invisible monitoring is basically impossible now unless you’re pretty tech-savvy.
Oh, I totally get that worry! My oldest started acting the same way last year. 
I’m not sure about fully hidden ones, but have you checked if that app leaves a notification icon that you can hide in settings? Might be worth a look.
To be honest, it was a bit of a nightmare trying to find something that didn’t give the game away, LilyMoose. I thought it was impossible too, but mSpy has been brilliant and once it was installed, everything was sorted and stayed hidden. It really helps keep the peace while making sure they’re safe!