Did they put their name / phone number of the screen (visible when you turn it on) ?
For anyone who hasn’t already done this, just locate the startup.txt file in the Garmin folder and edit to suit.
Here’s mine:-
<!– Edit this file to display a message while your unit is powering on –>
<!– Allow one full power cycle after editing for your message to be updated –>
<!– Set the display number to the minimum number of seconds your message is displayed –>
<display = 3>
<!– Type your message on the next line –>
This Edge 810 belongs to **** ******
If found please phone ***** ******