What an absolutely gorgeous girl and feral family! My heart goes out to you and your mum about Mummy. The loss of a feral can be especially heart breaking because there's only so much we can do to help them and then we question ourselves just as you have done for not finding her on the grass. Please don't do that to yourself. You and your mum gave Mummy the best possible life under the circumstances. She and her fur family knew the touch of human kindness which is more than a lot of ferals will ever know in their lifetime. You both have done a wonderful job in taking care of them but it's a dangerous world out there for cats. RIP, Mummy and peace to your humans.