Great mysteries. It was ruled an "accidental death", although allowing for the possibility of suicide. There was no note, though, and her friends had all said she had been in good spirits right before.
I prefer Simon Louvish's explanation from his sparklingly witty and wonderful 1999 Marx Brothers biography, Monkey Business.
A 'well-informed' and impeccable Hollywood source informs me, however, that the entire affair had been a cover-up from the start: Thelma had been giving her lover, director Roland West (1887-1952), oral attention in the car after a rumbustious night, but nipped his organ with her teeth, causing him to depart in high dudgeon and shut the garage door after him. She fell asleep in the car with the engine left running, and suffocated in her drunken stupor ...
Ms Todd has always been one of my favorite Pre-Code Goddesses, and somehow this salacious version of the tale, I think, would have satisfied her, too.
"You're a woman who's been getting nothing but dirty breaks. Well, we can clean and tighten your brakes, but you'll have to stay in the garage all night."