Air carrier Captain reported upon destination arrival that the Captain's NOTOC copy was discovered missing. After extensive investigation the missing document was not located.

Narrative

While parked in ZZZ1 and during our departure briefing; a loading staff member interrupted me with the Dangerous Goods Notification to Captain (NOTOC). I mentioned to my First Officer that we had 100lbs of dry ice onboard and I verified the information and signed both copies and handed them back to the loading staff. We completed our briefing and performance verifications and since we were close to scheduled block out time; I closed the main cargo and crew entrance doors. After an uneventful flight to ZZZ and after parking and completing checklists; I opened the main cargo and crew entrance doors. A staff member came up the stairs and asked if they were positioned properly and if we had a NOTOC. I retrieved the dangerous goods envelope from its storage pouch and noticed the NOTOC was missing. We searched the pouch and other areas and couldnt locate it. I tried calling ZZZ1 Operations with no answer. I also reached out to ZZZ and ZZZ2 Chief Pilots offices and notified them as well. Procedurally; after signing the NOTOC; the loading staff retains one copy and places the other copy in the DG Envelope and storage pouch. Unfortunately; this did not happen.