TL;DR: The concern here isn't a symptom you'll notice — it's that an old phone, tablet, or laptop sitting in a drawer or already sold/donated likely still has personal data on it unless it was properly wiped. On iPhone use Settings > General > Transfer or Reset iPhone > Erase All Content and Settings; on Android use Settings > System > Reset options > Erase all data; on a computer, use the built-in disk-erase utility with encryption enabled beforehand so any residual data is unreadable.
The concern here isn't a symptom you'll notice — it's that an old phone, tablet, or laptop sitting in a drawer or already sold/donated likely still has personal data on it unless it was properly wiped.
What Usually Causes This
Simply deleting files or doing a basic "restore to factory settings" on some older devices doesn't always fully erase the underlying storage, and many people skip the wipe step entirely before disposing of a device.
What to Do About It
On iPhone use Settings > General > Transfer or Reset iPhone > Erase All Content and Settings; on Android use Settings > System > Reset options > Erase all data; on a computer, use the built-in disk-erase utility with encryption enabled beforehand so any residual data is unreadable.
If this looks similar to something else you've seen, it's worth reading How to Check If Your Old Email Address Is Still a Weak Point.
This pattern shows up elsewhere too — see How to Check If Your Public WiFi Session Was Compromised.
Frequently Asked Questions
What are the real warning signs?
The concern here isn't a symptom you'll notice — it's that an old phone, tablet, or laptop sitting in a drawer or already sold/donated likely still has personal data on it unless it was properly wiped.
What usually causes this?
Simply deleting files or doing a basic "restore to factory settings" on some older devices doesn't always fully erase the underlying storage, and many people skip the wipe step entirely before disposing of a device.
What should I do about it?
On iPhone use Settings > General > Transfer or Reset iPhone > Erase All Content and Settings; on Android use Settings > System > Reset options > Erase all data; on a computer, use the built-in disk-erase utility with encryption enabled beforehand so any residual data is unreadable.
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.