The watchdog expires and gives no warning - just the system resets.

I'm currently well bitten by the "Dog". It took some detailed diagnosing in 0.27.5 that it was that subsystem when there are some cases in the SPI-12.
I introduced my own notifications on WatchDogAvr.h and it has since worked well to give me the status of the "Dog" as its petted.

I'd be happy to submit those if its of interest.