What sync actually copies.
Signing into a browser with an account, rather than just using it, turns on sync by default in most cases, and sync copies far more than most people expect. History, open tabs, saved passwords, autofill entries and bookmarks all travel between every device signed into that same account, updating within seconds of a change on any one of them. A tab opened on a phone genuinely does appear in the same browser's tab list on a laptop minutes later.
This happens quietly, with no confirmation step asking whether a specific tab or search should be shared onward, because sync is designed to feel seamless rather than to pause and ask each time. That seamlessness is exactly what makes it worth understanding properly, since the feature is doing far more in the background than the single sign in step that switched it on.
Where this becomes a problem.
The risk is not the syncing itself, it is forgetting which devices are signed in. A browser left signed in on an old phone, a work laptop returned to an employer, or a tablet a family member also uses can all still be receiving the same synced history and open tabs long after you stopped thinking about that device. Open tabs syncing is the most visible version of this: a tab opened privately on your phone can genuinely appear in a browser's recently closed or open tabs list on a shared family computer.
Checking which devices are actually signed in.
Chrome lists every signed in device under Settings, then the account section, with an option to sign out of any device remotely without needing to hold it. Firefox offers the same under Settings, then Sync, listing connected devices with a remote disconnect option beside each one. Reviewing this list once, particularly after a device is sold, lost or handed back, closes a gap that otherwise sits open indefinitely.
Scoping sync down rather than switching it off.
Most browsers let you choose exactly what syncs rather than an all or nothing toggle. Turning off tab sync specifically, while keeping bookmarks and passwords syncing, is often the single most useful adjustment, since open tabs are the category most likely to surface something unexpectedly on a screen you were not thinking about. This pairs well with separate browser profiles, since a personal profile can sync fully while a work profile stays local to one device only.
A companion habit worth adding.
Once a quarter, alongside the routine covered in clearing browser history, take the extra minute to open the device list and sign out anything unfamiliar, closing tab sync entirely if it has never once proved useful for the way you actually browse. It is a small habit that removes one of the more quietly persistent gaps in an otherwise well managed setup.
What happens when a device is lost rather than simply old.
A lost or stolen device with sync still active is a different situation from an old one gathering dust, since whoever has it can potentially see synced tabs and history updating in real time until it is signed out remotely. Acting quickly matters here: signing the device out of sync from another one you still control, then changing the account password as a second step, closes the gap faster than waiting to physically recover the device itself.
Most account providers also offer a way to see the device's approximate last known location alongside the sign out option, useful context when deciding how urgently to act, though it should be treated as a rough indicator rather than a precise one. Turning on two step verification for the account itself, covered in a separate guide on messaging apps but equally relevant to a browser account, adds a further layer that makes a stolen device alone insufficient to access sync in the first place.
Where this matters most.
This matters most around anything tied to business events arranged near Amsterdam Noord or a work device more broadly, where a tab reading through companions filed under brunette or blonde syncing onto a signed in work laptop is exactly the scenario worth preventing before it happens rather than after. Checking rates from a personal profile with tab sync scoped down avoids the issue entirely, regardless of which other device happens to be signed into the same account.
Treat the device list review as a companion task to the profile split covered separately, since the two together close most of the gap: one keeps work and personal browsing from mixing in the first place, the other makes sure nothing keeps quietly syncing to a device that no longer needs it. Doing both once, then revisiting the device list every few months, covers the situation thoroughly without turning sync management into an ongoing chore, and both take less time in total than a single afternoon spent tidying up after the fact would.