B767 pilot reported receiving several indications of right engine malfunction during after landing taxi. After engine shut down; maintenance advised that a piece of right inboard engine cowling was missing.

Narrative

Landed on Runway XXL in ZZZ. Upon touchdown I observed all normal indications (speed brakes up and reversers green). Reverse thrust was selected and I did not observe nor did I feel anything unusual or abnormal throughout the landing rollout. Upon clearing Runway XXL and after receiving our taxi clearance; I began my after-landing flow and then first noticed something non-normal. What I first noticed at that point was an IDLE DISAGREE EICAS message and an amber right REV. Upon further examination I observed that the right N1 was slightly higher (35.8%) than the left N1 (26.7%) with both engines at idle. There was a slight delay upon entering the ramp; and with the parking break set; the First Officer referenced the 767 QRH procedure for IDLE DISAGREE. The QRH did not instruct us to shut down the engine so I felt comfortable taxiing to our parking assignment with the engine running. Upon block-in and after the Shutdown and Secure checklists were complete; I proceeded to make the appropriate entries/write-ups in the aircraft's logbook. It was at that time that aircraft Maintenance met us in the cockpit and informed us that a piece of the right inboard engine cowling on the right engine was missing. I immediately contacted ZZZ Tower; relayed to the controller what Maintenance had told us; and advised the controller of possible FOD/debris on Runway XXL. After that I completed making the logbook entries. In addition to the entries regarding IDLE DISAGREE and the amber REV; I also made an entry regarding the engine cowling at the request and guidance of the aircraft mechanic. At that point I handed the aircraft over to aircraft Maintenance and departed the aircraft.