As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
EN ROUTE TO YUL, A-SYSTEM HYDRAULIC QUANTITY DECREASED TO 1.0 GALLON DURING GEAR/FLAP RETRACTION. AFTER RETRACTION WAS COMPLETED, QUANTITY INDICATED 1.5 GALLONS. FLIGHT RETURNED TO DTW WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED A-SYSTEM QUANTITY TRANSMITTER AND SERVICED TO 4.2 GALLONS. LEAK AND OPERATIONAL CHECK NORMAL.