Hmm - whenever I've found a mountain of substrate it's usually on top of a hoard lol! To bury it. It sounds like she has been doing some housekeeping. Maybe she has discarded the stuff she put in the bowl. Sometimes I'll find a bit mound of substrate full of bits of food outside the front door of Charlie's house - he chucks it all out when he wants it taking away! I once found it piled up in front of the door and it was quite stinky so maybe he wee'd on it and decided he didn't want it in there any more.
It's fascinating what they get up to. I should just observe - she must know you empty the food bowl so I should carry on as normal. If she doesn't like that it has been taken away when you empty the food bowl and clean/refill as normal, then she won't put it there again! I suspect it is food she has discarded and kept the best bits under the mountain
