I'm so sorry to hear that. Same exact thing happened to my Rowdy. The neighbors called her "Red Death" Fastest little booger you ever saw, and she could jump so high that the neighbor gave up trying to raise his bird feeder high enough.
It was heart wrenching when I lost her. No sign of anything. Just curled up under a neighbors rose bush and died. It was warm out, so there was no worry about freezing. Then her son Jimmy died a year or so later, same way. I think one of the neighbors was poisoning them.
I just feel for you. It's horrible