Not a bella donna that I thought it may be.. but could be a cousin possibly a kaffa/kaffer lily.. can't find any photos of them at mo.. pleased you are all ok after all your drama..
It reminds me of an Easter Lily but it's red. However your flowers are different from ours, so I can't really offer a suggestion. I wonder how it ended up in your garden!
(Picture this - my app - says "Striped Barbados Lily; species of Amaryllis"...)
Ian