TL;DR: Warning signs on a shared computer include saved passwords or browsing history you don't recognize, unfamiliar user accounts, or one person's files/settings changing without their input. Set up a separate user account for each person (Windows Settings > Accounts > Family & other users, or macOS System Settings > Users & Groups), enable a password on each account, and use separate browser profiles so saved passwords and history stay private to each person.
Warning signs on a shared computer include saved passwords or browsing history you don't recognize, unfamiliar user accounts, or one person's files/settings changing without their input.
What Usually Causes This
This usually happens because a shared computer doesn't have separate user accounts set up, so everyone shares the same browser profile, saved passwords, and file access by default.
What to Do About It
Set up a separate user account for each person (Windows Settings > Accounts > Family & other users, or macOS System Settings > Users & Groups), enable a password on each account, and use separate browser profiles so saved passwords and history stay private to each person.
If this looks similar to something else you've seen, it's worth reading How to Secure Your Smart Baby Monitor.
This pattern shows up elsewhere too — see How to Secure a Smart Doorbell or Camera.
Frequently Asked Questions
What are the real warning signs?
Warning signs on a shared computer include saved passwords or browsing history you don't recognize, unfamiliar user accounts, or one person's files/settings changing without their input.
What usually causes this?
This usually happens because a shared computer doesn't have separate user accounts set up, so everyone shares the same browser profile, saved passwords, and file access by default.
What should I do about it?
Set up a separate user account for each person (Windows Settings > Accounts > Family & other users, or macOS System Settings > Users & Groups), enable a password on each account, and use separate browser profiles so saved passwords and history stay private to each person.
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.