TL;DR: Warning signs include messages showing as read before you've opened them, your device syncing texts to an iCloud or Google account you don't recognize, or a spike in data usage tied to your messaging app. Check Settings > [your name] > Devices (iPhone) or your Google Account's Security > Your devices (Android) for unrecognized sign-ins and remove them, enable two-factor authentication, and change your account password immediately.
Warning signs include messages showing as read before you've opened them, your device syncing texts to an iCloud or Google account you don't recognize, or a spike in data usage tied to your messaging app.
What Usually Causes This
This usually happens through an unauthorized sign-in to your iCloud or Google account on another device, a linked companion app (like Messages on a Mac or Android's message-forwarding feature) you didn't set up, or stalkerware installed directly on your phone.
What to Do About It
Check Settings > [your name] > Devices (iPhone) or your Google Account's Security > Your devices (Android) for unrecognized sign-ins and remove them, enable two-factor authentication, and change your account password immediately.
If this looks similar to something else you've seen, it's worth reading How to Tell If Someone Cloned Your SIM Card.
This pattern shows up elsewhere too — see How to Turn Off Ad Tracking on iPhone and Android.
Frequently Asked Questions
What are the real warning signs?
Warning signs include messages showing as read before you've opened them, your device syncing texts to an iCloud or Google account you don't recognize, or a spike in data usage tied to your messaging app.
What usually causes this?
This usually happens through an unauthorized sign-in to your iCloud or Google account on another device, a linked companion app (like Messages on a Mac or Android's message-forwarding feature) you didn't set up, or stalkerware installed directly on your phone.
What should I do about it?
Check Settings > [your name] > Devices (iPhone) or your Google Account's Security > Your devices (Android) for unrecognized sign-ins and remove them, enable two-factor authentication, and change your account password immediately.
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.