ThummerTheft Detection Lock Not Working? What to Check
You turned on Theft Detection Lock, tried a fake snatch in your kitchen, and nothing happened. Or the opposite: the phone locked itself on the bus. Neither means the feature is broken.
Theft Detection Lock works under specific conditions, and Google's own help page lists cases where it may not fire. Here's what to check, and what to add for the gaps it leaves.
What Theft Detection Lock is meant to do
Google announced the feature in October 2024 alongside Offline Device Lock and Remote Lock. It uses on-device machine learning and the phone's sensors to spot a likely theft from an unlocked phone, then locks the screen. Google said the features would reach "most Android 10+ devices" through a Google Play services update, and they are not supported on Android Go phones, tablets or wearables. For a tour of every built-in feature, see our guide to Android theft protection, explained.
It does one thing: it sends the phone back to the lock screen. An Android Authority writer who tested it in 2025 put it plainly: after it fires, your PIN or pattern still unlocks the phone normally, so the rest is up to you.
Check 1: Is it on, and is your phone supported?
On most Android phones, open Settings, tap Google, then All services, then Theft protection, and turn on Theft Detection Lock.
On Samsung phones with One UI 7, Samsung lists it under Settings > Security and privacy > Lost device protection > Theft protection, next to Offline device lock, Identity check and Remote lock. On One UI 6.1 or earlier, Samsung points to the Google path instead: Settings > Google > All services > Theft protection.
If the switch is grayed out, Google says your device doesn't support the feature. Google also said at launch that the rollout was gradual, to ensure compatibility with different devices.
In some places it may already be on. In January 2026, Google said that on new Android devices activated in Brazil, Theft Detection Lock and Remote Lock are now enabled by default. Elsewhere, check the setting yourself.
Check 2: Was the phone unlocked and in use?
Google's help page says that when the screen is locked, Theft Detection Lock isn't active. That makes sense: a locked phone is already locked.
There's a second, less obvious condition. The Android Authority tester found that detection seemed to need an app in active use. He couldn't get it to trigger with the phone idle on the home screen. That's one person's testing, not a Google statement, but it matches the feature's purpose: a phone grabbed out of your hand mid-use.
Check 3: Was it connected to Wi-Fi or Bluetooth?
To limit interruptions, Google says Theft Detection Lock may not trigger when your phone has a stable Wi-Fi connection, a Bluetooth connection, or both. If you tested it at home on your own Wi-Fi, or with earbuds or a watch paired, that alone could explain a no-show.
Check 4: Did you "steal" it the way a thief would?
The Android Authority writer ran with his phone for about 15 seconds and nothing happened, and many more attempts didn't lock it either. Google told him the feature relies mainly on the accelerometer, looking for a sudden movement or change in direction. His takeaway was that real thieves run longer and faster than he did, and he did eventually get it to trigger.
Google's help page describes the target case the same way: if someone grabs the phone out of your hand and runs, bikes or drives away, the feature "may" activate. It isn't a promise for every snatch.
Check 5: Did it already lock recently?
Google says the feature may not trigger after repeated locks in a short period. The tester observed that it stops after two consecutive locks, which he understood as a guard against false positives. If you tested it twice in a row, a third try may do nothing.
The opposite problem: it locks when nothing happened
Some owners have the reverse complaint. In April 2025, Android Authority reported that users were seeing the feature lock phones during everyday activity, such as running or sitting on a bus. The site said Google had told it the feature mainly relies on the accelerometer to sense sudden movements or changes in direction, which could explain it. The site suggested the sensitivity may have been raised since launch, but that was its inference, not something Google confirmed.
If false locks bother you, the fix is usually just your normal unlock. Turning the feature off removes a free layer of protection, so weigh that before you do.
Where it leaves gaps
Even when it works perfectly, Theft Detection Lock has blind spots:
- The snatch with no run. The tester pointed out that if someone grabs your phone as a train door closes, they don't have to run at all.
- A phone on the table. A phone sitting idle and taken while you aren't looking is unlikely to trigger it.
- A known PIN. If someone watched you type your PIN, they can unlock the phone after it locks. Some of the cases in our piece on how thieves drain bank apps from a stolen phone started exactly that way.
- Connected phones. The Wi-Fi and Bluetooth limit above.
What to turn on alongside it
Theft Detection Lock is one part of a set. On the same settings page:
- Offline Device Lock locks the screen after the phone goes offline. The phone must be unlocked when it loses its connection.
- Remote Lock lets you lock the phone from android.com/lock using its phone number. It must be turned on beforehand, and if the phone is offline, it locks when it reconnects.
- Identity Check asks for your fingerprint or face for sensitive actions, such as changing your screen lock or running a factory reset, when you're away from trusted places.
- A strong, private PIN. It's what everything above falls back to.
Some people add a layer for the moment of the grab itself. A wrist strap is the low-tech version, and it's the Android Authority writer's own advice. Third-party apps take other approaches. Thummer, for example, is one option: if the owner's thumb lifts off an on-screen bubble, the screen goes black, a fingerprint check runs, and the phone locks within five seconds. Whatever you add, keep Google's built-in protections on underneath it.
The bottom line
If Theft Detection Lock didn't fire, check five things: the setting is on and supported, the phone was unlocked and in use, it wasn't on stable Wi-Fi or Bluetooth, the movement looked like a real snatch, and it hadn't just locked twice. Then turn on Offline Device Lock, Remote Lock and Identity Check, so one feature's limits don't become your problem. And if the worst happens, our first-10-minutes checklist covers what to do, in order.