Whoever did this had better hope the police catch him before the neighbors do. This clown could get lynched, or have his garden burned down.
If you live in a densely populated area, which I assume this is, you have all kinds of considerations that you must abide by to keep the status quo. Keeping your cats off of someone else’s property might be one of those things to do. ;-)
...or have his garden burned down.
If my cat was missing and I received such a letter, it would be more than the garden burning.