Is there any ominous sign that a turtle with a turtle neck died on the fence of my window?

Don't believe any signs. Since your grandmother often feeds you, maybe she thinks your place is safer, so she came anyway. I just didn't expect to end up here. Maybe this is a treasure house. You see, bury it in the grass or flower bed around your home. I buried many birds and cats around my house. They all died of natural causes. I don't need to think much.