I don't know. I believe check in msg should have no payload, but it usually does have a u32 of unknown purpose. I have found that the very first check-in after power up has no payload according to candump and this was causing alarm 46. Until UI sorts this out, I am not going to check payload.
I saw the corresponding alarm 44, on the initial power-up. However, going back to the UI log file shows UI communication within 1 second of triggering the error. So either the received message was that much delayed, or the check is otherwise broken.