TL;DR: There's no on-device "symptom" here — the issue is simply that ad tracking is enabled by default on both platforms, meaning apps and advertisers can build a profile of your activity across other apps unless you turn it off. On iPhone, go to Settings > Privacy & Security > Tracking and turn off "Allow Apps to Request to Track," then also disable Settings > Privacy & Security > Apple Advertising > Personalized Ads. On Android, go to Settings > Privacy > Ads and select "Delete advertising ID" or opt out of ad personalization.
There's no on-device "symptom" here — the issue is simply that ad tracking is enabled by default on both platforms, meaning apps and advertisers can build a profile of your activity across other apps unless you turn it off.
What Usually Causes This
iOS uses an Identifier for Advertisers (IDFA) and Android uses a Google Advertising ID, both of which are active out of the box and used by ad networks to track behavior across apps for targeted advertising.
What to Do About It
On iPhone, go to Settings > Privacy & Security > Tracking and turn off "Allow Apps to Request to Track," then also disable Settings > Privacy & Security > Apple Advertising > Personalized Ads. On Android, go to Settings > Privacy > Ads and select "Delete advertising ID" or opt out of ad personalization.
If this looks similar to something else you've seen, it's worth reading How to Check If a USB Drive Is Safe to Plug In.
This pattern shows up elsewhere too — see Is My Android Phone Hacked? Signs to Check For.
Frequently Asked Questions
What are the real warning signs?
There's no on-device "symptom" here — the issue is simply that ad tracking is enabled by default on both platforms, meaning apps and advertisers can build a profile of your activity across other apps unless you turn it off.
What usually causes this?
iOS uses an Identifier for Advertisers (IDFA) and Android uses a Google Advertising ID, both of which are active out of the box and used by ad networks to track behavior across apps for targeted advertising.
What should I do about it?
On iPhone, go to Settings > Privacy & Security > Tracking and turn off "Allow Apps to Request to Track," then also disable Settings > Privacy & Security > Apple Advertising > Personalized Ads. On Android, go to Settings > Privacy > Ads and select "Delete advertising ID" or opt out of ad personalization.
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.