TL;DR: Signs include apps you don't remember installing, pop-up ads appearing outside of any app, the phone overheating or losing charge unusually fast, and higher-than-normal mobile data consumption. Open Settings > Security & Privacy and run a Google Play Protect scan, review Settings > Apps for anything unfamiliar and uninstall it, check for unrecognized device administrators under Settings > Security, and factory reset from Settings > System > Reset options if the problem doesn't clear.
Signs include apps you don't remember installing, pop-up ads appearing outside of any app, the phone overheating or losing charge unusually fast, and higher-than-normal mobile data consumption.
What Usually Causes This
Most cases come from a sideloaded app outside Google Play, a malicious APK granted broad permissions, or an old version of Android missing recent security patches.
What to Do About It
Open Settings > Security & Privacy and run a Google Play Protect scan, review Settings > Apps for anything unfamiliar and uninstall it, check for unrecognized device administrators under Settings > Security, and factory reset from Settings > System > Reset options if the problem doesn't clear.
If this looks similar to something else you've seen, it's worth reading Is My iPhone Hacked? Signs to Check For.
This pattern shows up elsewhere too — see Is My Laptop Hacked? Signs to Look For.
Frequently Asked Questions
What are the real warning signs?
Signs include apps you don't remember installing, pop-up ads appearing outside of any app, the phone overheating or losing charge unusually fast, and higher-than-normal mobile data consumption.
What usually causes this?
Most cases come from a sideloaded app outside Google Play, a malicious APK granted broad permissions, or an old version of Android missing recent security patches.
What should I do about it?
Open Settings > Security & Privacy and run a Google Play Protect scan, review Settings > Apps for anything unfamiliar and uninstall it, check for unrecognized device administrators under Settings > Security, and factory reset from Settings > System > Reset options if the problem doesn't clear.
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.