I just started working with a Mega. When I first powered it up LED_BUILTIN was blinking.
My first thought was that possibly it comes with the 'blink' sketch loaded as a check to see if it functions.
I uploaded an empty sketch to clear out anything that may be running.
Now it is no longer blinking, but it is on steady.
I can easily digitalwrite it low in each sketch to turn it off, but I am concerned this is possibly a symptom of something else.
I have read others saying they have it light dimly, mine lights bright.
I did notice that when I try to measure the voltage between 13 and gnd (about .3 volts) they LED does turn off, but turns right back on when I remove the multi meter.
Any reason for concern?