We've lost the odd one the same way too.
Had one last year that laid in a rut like that "6 or so deep. By the time we found her and got her out we figured she had been there for well over 12 hours. The Co-op fuel truck driver had seen her in the morning (10 am) and thought she was dead. We saw him that night at a supper(6 pm) and he asked about her. I thought he was joking, but we went out to check and sure enough there she was (right along the main grid). DH saw her move a bit, or we would likely not have even gone over to her, and she really would have died.
She was bloated right up, had prolapsed her rectum, and her head was swollen. As soon as we sat her upright the bloat started to go down, we pushed her rectum back in, and it took a few days for her head to go back to normal. She calved about 3 weeks later, it was a good healthy calf too. But she came up dry last fall....