TL;DR: Signs to check first include unfamiliar apps in your app list, unusually high battery or data usage tied to an unknown app, and accessibility or device-admin permissions granted to something you don't recognize. Run a Google Play Protect scan from Settings > Security & Privacy, check Settings > Apps > Special Access > Device Admin Apps and revoke access from anything unfamiliar, uninstall the suspicious app directly or boot into Safe Mode to remove it if it resists uninstalling, and if it persists, back up your data and perform a factory reset from Settings > System > Reset options.
Signs to check first include unfamiliar apps in your app list, unusually high battery or data usage tied to an unknown app, and accessibility or device-admin permissions granted to something you don't recognize.
What Usually Causes This
Spyware on Android is most often sideloaded from outside the Google Play Store, sometimes disguised as a system app or hidden from the app drawer to avoid detection.
What to Do About It
Run a Google Play Protect scan from Settings > Security & Privacy, check Settings > Apps > Special Access > Device Admin Apps and revoke access from anything unfamiliar, uninstall the suspicious app directly or boot into Safe Mode to remove it if it resists uninstalling, and if it persists, back up your data and perform a factory reset from Settings > System > Reset options.
If this looks similar to something else you've seen, it's worth reading How to Remove Spyware From an iPhone.
This pattern shows up elsewhere too — see How to Secure a Home Security Camera System.
Frequently Asked Questions
What are the real warning signs?
Signs to check first include unfamiliar apps in your app list, unusually high battery or data usage tied to an unknown app, and accessibility or device-admin permissions granted to something you don't recognize.
What usually causes this?
Spyware on Android is most often sideloaded from outside the Google Play Store, sometimes disguised as a system app or hidden from the app drawer to avoid detection.
What should I do about it?
Run a Google Play Protect scan from Settings > Security & Privacy, check Settings > Apps > Special Access > Device Admin Apps and revoke access from anything unfamiliar, uninstall the suspicious app directly or boot into Safe Mode to remove it if it resists uninstalling, and if it persists, back up your data and perform a factory reset from Settings > System > Reset options.
Should I be worried if I only see one sign?
One sign alone (like a slower device) is usually not cause for alarm — most single symptoms have mundane explanations. Multiple signs together, especially unexpected data usage or unfamiliar apps, are worth investigating.