I was alerted to a failure to pressurize in a King Air by a packet of potato chips (crisps in the UK :D) that burst going through 16,000′ for a final FL210. IIRC the only warning is a yellow light on the annunciator panel (see image below). Always carry an unopened packet of potato chips with you, it may save your life. I’ve also flown the 421, max FL260, but in those days I was fearless :diablo:

RIP and condolences to the family 🙁