[Retroman1969]

From my experience, these cooling systems need to be carefully and thoroughly bled after a flush, and are incredibly fussy about it. The low coolant flashing light is also unpredictable, as sometimes it will flash while the level is above the mark and not flash when the level is well below the mark.
I had a similar issue after a flush on my '99.
Every few days the light would start flashing and I had to top up the level. I took the car back to the shop several times over this period where the whole system was throughly checked and no leaks or compression issues were found. I replaced the coolant cap for good measure, but it didn't change the behavior.
So I just resolved myself to topping up a couple of times a week until it finally stopped flashing and the coolant level stopped mysteriously dropping. It took about 4 months and went through about a gallon of coolant. That was 6 months ago. Have had no issue since.