If its not oil or antifreeze, then it could be battery overflow due to charging. If it is, RINSE WELL WITH WATER AND BAKING SODA.
I believe John may be correct. Battery is Gel ( I think)jzeiler said:That is just overflow from the wheel circuit reservoirs. There may have been a temperature difference in the shop and outside when they flushed the brakes. That was expansion. I usually set the level with the caliper pistons fully retracted then pump them back out. If they pumped them out and THEN set the level it would explain the overflow. If you feel that "water" it will seem slimy on your fingers. Don't get any on a painted surface.