Mine did a similar thing. The coolant drain plug is on the lower left of the radiator (when on the bike). Worth asking what the shop actually did, as there is a coolant drain bolt on the engine block that supposedly you should undo, but it is a pain to get to.
It might be just bad luck, as one of the hoses under the carbs may have leaked (sometimes just doing up the clamps fixes it), or like mine one of the alloy elbows under the carbs has cracked. During a standard coolant change none of the hoses or elbows under the carbs would have been looked at. At 130,000kms it would be best to replace the hoses under the carbs and the allow elbows. The smaller bypass hose on mine had become really brittle, and would have failed sometime. Carb removal is not that hard, but may be daunting depending on your level of confidence.
One think to check is the radiator cap (top right of the radiator under the right side pocket) as I have had a perished radiator cap seal, and the overflow hose that goes to the coolant overflow bottle is well known to crack and leak.
And just to put things in perspective, I have had cars with far more coolant loss, thermostat hoses going etc problems than the ST11.
