What it means
Android's automatic unknown-tracker alert is designed to flag a compatible tracker separated from its owner that appears to move with you over time. A manual scan checks for compatible separated trackers currently nearby. These answer different questions and can produce different results.
A real-world example
A manual scan finds a tag while you are in a crowded café. It may be someone else's misplaced item and does not establish that it traveled with you. An alert with a movement map provides a stronger reason to investigate your belongings and route.
What to do
Open the alert promptly, save its history, use the available sound or identification tools and move to a safe location if worried. A manual scan can supplement, not replace, an alert.
The distinction that matters
No result proves there is no tracker: range, owner proximity and compatibility matter. Google's safety page explains why an alert may be delayed and why a tag may not ring.
Google's tracker-alert help
