Air carrier Captain reported the undocumented transport of dry Ice due to missing NOTAC at departure gate. The Captain completed an exterior post flight inspection and was able to confirm that dry ice was in fact loaded without flight crew notification.

Narrative

I was called on reserve to operate flight ZZZ to ZZZ1. The flight had diverted the night before; because of this the rampers said they would have to provide me with a paper report; which they did. I took the bottom copy and they kept the top copy. On the report there was 19 pounds of cargo noted for the FWD cargo. I was not provided with a NOTAC or advised of any DG (Dangerous Goods) or dry ice when the ramper came up to talk to me. Upon arrival in ZZZ1 I received an ACARS message from dispatch saying ZZZ ops had just called them and advised them that dry ice had been loaded onto the aircraft without PIC notification. Ops told dispatch that they could not confirm how much dry ice was loaded or which cargo bin it was placed in. When I was made aware of this I went outside while bags were being unloaded and confirmed that dry ice had been loaded into the FWD cargo. I identified a box clearly labeled with [shipping label] UN1845" and "dry ice 4kg". A separate incident report had also been filed. Suggestions: Anytime I see cargo on the report; I will verify what it is with the rampers."