A yellow LED is flashing on the BrickPi3, no red LED - what does that mean?

when I mount the BrickPi3 on my Pi then a yellow LED is flashing (between GPIO header and the Brickpi3 power plug, about 3-4 times/sec.):

what does that mean?

There is no red LED on at BrickPi3, just on the RaspberryPi…

(power switch is “on” but no external batteries attached because I don’t plug motors FTM)

See here for details regarding the BrickPi3 yellow LED flashing.

alright, thanks, now I got it.
It looked like an urgent emergency signal, and due to the fact the there was no connection to the BrickPi3 I must admit that urgent emergency signals make me nervous… :-/