CRJ-900 flight crew reported non-adherence to MEL altitude restriction.

Narrative

Upon receiving the aircraft from the inbound crew; we double checked the CDL with the release and maintenance lock book and found the door in the closet; the FO didn't mention anything unusual about the door missing and I didn't think to ask about the plug ;everything seem to be in order and we continued the flight for another three legs going from ZZZ to ZZZ1 to ZZZ2 and back; I remember one of the ground crew on one of the legs I believe ZZZ2 coming up to the flight deck with a photo of that area and ask if we knew the door was missing and showed the photo with the plug missing however I didn't think it unusual at the time; last time I had a plane that the plug missing we ended up taking another aircraft and that was at least a year or more ago; today a crew member informed me about the delay on that aircraft due to realizing it was missing the MEL and should've been limited to 25;000 feet. I will now try to be much more aware of that in the future.Cause: There was definitely a breakdown of procedure somewhere along the line given the crew that mentioned it to me said when maintenance was initially informed of it; they also didn't find it unusual for the plug to be missing; both initial maintenance working on it and the previous crew who had the plane before us didn't think anything was unusual about it; and the MEL has rather specific wording forgoing mentions of low and pressure in favor of conditioned air.Suggestion: Perhaps an added note to the CDL item to make sure the condition of the plug is mentioned; from what I remember of the photo and the crew that informed me of the missing MEL it looked like the door had been intentionally removed at some point; but it could've also been pulled off inflight; the MEL for that plug doesn't mention low pressure air and instead refers to it as conditioned air thus it took me about five minutes of searching for it after I knew what I was looking for to even find it.