Emptying rear brake reservoir but no leak

Good day!

Has anybody experienced this? Bike got serviced at dealership. They changed fluid and pads. After some time noticed the rear brake is a bit off. Opened the reservoir and it's empty. So topped it off checked for signs of leak (there was none) and everything was back to usual. Then again after some time (no data on milage or time) it's emty again. They used Motul in the dealership. I used Prestone Synthetic. Bike is used often in heavy city traffic.

Since the dealer did the service on it, take it back to them & have them fix it. Brake fluid isn't thinners, it doesn't evaporate, it's going somewhere, have them sort it at their cost.