PC-12 pilot reported Cabin Pressurization Warning and return to departure airport.

Narrative

Aircraft Autopilot was engaged and just captured cruise altitude of 26;000 ft MSL. Not long after; I looked at the CAWS (Central Advisory and Warning System) Panel and saw that the CAB PRESS light was illuminated. I glanced at the pressurization gauge and found that the cabin altitude needle was well over 11;000 ft and the differential needle was over 5.75 psi. The master warning light did not illuminate nor was there a aural chime to alert the pilots there was a problem. I had my student co-pilot search for the checklist while I asked ZZZ Center for clearance to descend lower and return to our departure airport (ZZZ) as a precaution. I did not request priority handling. Once I was cleared to descend and return to ZZZ; I completed a 180 degree turn towards ZZZ and pitched down to roughly -3500 fpm. I instructed my co-pilot to don his oxygen mask as a precaution while I continued to descend. Reaching 12;000 ft MSL; the CAB PRESS light extinguished and the aircraft seemed to function normally. Shortly there after; I landed the aircraft safely in ZZZ. The indications and situation that caused the CAB PRESS light to illuminate are confusing; contradictory; and uncertain. The aircraft was inspected and completed a Maintenance test flight following the air return; unable to duplicate the issue. Theories on what may have caused this issue were a potential overheat of the ECS (Environmental Control System) system; or perhaps; a sticky valve of the pressurization system.