rigamortis in cats

I found my 15 yr old cat dead on his chair on the back verandah outside this morning. He was very stiff. Anyone have an an idea when he may have died? No fluids were apparent and his eyes were open and dilated. He had a good and long life, but I'd like to know when he died. Must have had heart failure. No signs or symptoms. He looked very comfortable in the position he died in, the position he was always in on his chair - at first I thought he was just resting. He was on my lap at mid-night last night with no distress. No body wounds. Just a shame I wasn't with him but it may have happened in seconds.