I agree, I would transalte this sentence as 'she had left the keys at home'

I don't see any problem with it. "i forgot my keys" "Where?" "At home" "You forgot your keys at home?!"

Unusual, and far-fetched: "Mum had lost the keys on the hillside and thought we were stuck. She had forgotten the keys at home. It was a duplicate set. I went back for them". Possible.