The secret conversation is designed to display a lot of things. One of the many things that you’ll notice when using Facebook messenger’s secret conversation feature is “first seen by this device”. But what exactly does that mean?
Well, “first seen by this device” is a feature in the secret conversation; it tells you exactly when the person in question (the owner of the device) first enabled the secret conversation toggle button.
As earlier mentioned, when joining the secret conversation for the first time, you need a Messenger app. From the app, you need to locate the secret conversation feature, which usually comes with a lock icon. To enable the feature, you need to tap the toggle switch in the secret conversation.
The “first seen by this device” is often displayed to show exactly when a user tapped the toggle switch for the first time to enable the secret conversation feature on the device
You’ll see a date beside the “first seen by this device;” that’s the exact date the feature was first enabled on the device.
The bottom line is that the “first seen by this device” is a feature that notifies you of when a device first enabled the secret conversation feature.