I don't recall whether this has come up before.
A week or two ago, when I was using my '33S, I got a low battery warning but everything still worked. I then turned it off (I'm pretty sure).
Two days ago, when I went to turn it back on, I got absolutely no indication that it was alive. Fortunately, nothing of much value was stored.
Today I measured the battery voltages - they were about as low as I've ever seen for batteries! Less than 0.1 V for either, no load. I'm rather surprised that the calculator in the OFF state could pull them so low. I wonder if something might have failed internally. I don't have new ones to put in yet to check that possibility. That's tomorrow's task.
So, a warning: if your '33S indicates low batteries, you better change them right away if you want to retain your work.