TL;DR: As with location, the real red flag is usually an app having contacts access that has nothing to do with its function — a flashlight or game app requesting your full contact list is a common example. On iPhone, go to Settings > Privacy & Security > Contacts and revoke access from any app that doesn't need it. On Android, go to Settings > Apps > [app name] > Permissions > Contacts, or use Settings > Privacy > Permission manager to review every app at once.
As with location, the real red flag is usually an app having contacts access that has nothing to do with its function — a flashlight or game app requesting your full contact list is a common example.
What Usually Causes This
Apps often bundle a contacts-access request into a broad initial permissions prompt, and most users tap "Allow" without reviewing whether the app actually needs it.
What to Do About It
On iPhone, go to Settings > Privacy & Security > Contacts and revoke access from any app that doesn't need it. On Android, go to Settings > Apps > [app name] > Permissions > Contacts, or use Settings > Privacy > Permission manager to review every app at once.
If this looks similar to something else you've seen, it's worth reading How to Check What Apps Have Access to Your Location.
This pattern shows up elsewhere too — see How to Check What Apps Have Access to Your Microphone.
Frequently Asked Questions
What are the real warning signs?
As with location, the real red flag is usually an app having contacts access that has nothing to do with its function — a flashlight or game app requesting your full contact list is a common example.
What usually causes this?
Apps often bundle a contacts-access request into a broad initial permissions prompt, and most users tap "Allow" without reviewing whether the app actually needs it.
What should I do about it?
On iPhone, go to Settings > Privacy & Security > Contacts and revoke access from any app that doesn't need it. On Android, go to Settings > Apps > [app name] > Permissions > Contacts, or use Settings > Privacy > Permission manager to review every app at once.
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.