I'm assuming you asked all your neighbours with garages/sheds? my cat went missing once and I found him in a detached garage about 4 houses down. Curious bugger so the owner never even saw him go in there and she locked him inside it.

My other cat went missing, but she voluntarily left the house where they were looking after her while I went to another city to get myself established first. They told me she jumped out of their 3rd floor apartment window and since things weren't working out in the new city so I jumped on the next plane and headed to that area straight from the airport. I kept calling her name and in about 10 minutes she came out, all skinny (she was "missing" about a week). It was a very Disney-esque reunion.

I hope Tortilla comes back!