If she had shit herself recently, he would have noticed before trying to take her underwear down. That's the kind of stench that isn't contained by pants to begin with but the moment he removed the outer layer he definitely would have noticed the smell. Plus he said he encountered a stain, not a layer of actual shit. It is far more likely that she was wearing underwear that had been stained brown by period blood but the OP assumed it was a shit stain, than it is that he managed to undress her down to her underwear without noticing the stench of her having crapped her pants earlier in the day.

If she had a small bit of the down-town-brown left and missed it somehow he wouldn't have noticed the smell until he dove in, most likely.

He referred to it as "the mother of all shit stains" that doesn't seem like it would have been a "Little bit" of anything. Skid marks, which as I'm sure you know are actually the result of someone failing to wipe correctly, make underwear stink. If it was a new "mother of all shit stains" and not a stain from something that occurred before the underwear was last washed he would have definitely noticed the smell as soon as his head got anywhere near her lower half. There is no way he would have failed to notice actual shit on her underwear until after he had pulled her underwear far enough down, with his teeth, to see the inside of them unless he has zero sense of smell, which would be a bigger FML.