B737-800 Captain diverted after receiving HYD annunciation during gear retraction and consultation with Maintenance.

Narrative

Operating as the pilot flying during gear retraction; we received HYD on the RECALL panel. The PM looked to see what it was; but the light went out before he could verify the cause. We continued climbing and pressed the RECALL switch occasionally; without any issues. During cruise we decided to press the SYS page and noticed the HYD QTY on SYS A was 74%. We decided to periodically check and at one point it showed 71%. I decided to contact Maintenance Control through the EFB via dispatch. Maintenance Control advised me to call him if it drops again within 20 minutes. I acknowledged him; started a timer; and disconnected the call. Within 2 minutes the quantity in system A dropped to 70%. I contacted Maintenance Control again and it was determined we had a possible hydraulic leak and needed to divert. I was asked if we could fly to a company station such as ZZZ; ZZZ1; or ZZZ2; but they were all a farther distance. I told Maintenance Control and dispatch it was better to land in ZZZ3 since we were 80 miles away. I did not want to fly a farther distance with a possible hydraulic leak. I let dispatch know we were going to [request priority handling] and divert to ZZZ4. Before [requesting priority handling]; I transferred control and radios to the FO; then I spoke with the flight attendants; followed by the passengers. Afterwards I had the FO [request priority handling] and I ran the non routine landing considerations checklist. After finishing the checklist and briefing the approach; control was transferred back to me. We landed flaps 40 on [Runway] XXR in ZZZ4; and stopped on the runway to be inspected by ARFF (Airport Rescue and Firefighting). While the ARFF vehicles inspected the aircraft; I spoke with the passengers to keep them in the loop. After we were cleared by ARFF; we taxied to the gate under our own power; parked; and wrote up the aircraft.Cause: Possible hydraulic leak and monitored the SYS page after receiving HYD on RECALL panel. To mitigate risk we continued monitoring the hydraulics and contacted dispatch and Maintenance Control.