LOW COOLANT?

I have an '02 Grand Cherokee with a 4.0 engine.
I just had the anti-freeze flushed/drained
and re-filled a week ago. Currently the overhead
console flashes "low coolant" constantly!!
Is there a coolant level sensor for this engine..
if so where is it located? Thanks for your help!

in overflow

thanks..off to O' Reilly's I go