Sorry to hear your rotten news..hope you get some answers..
This kinda happend to me a few years ago my cat kept going away for days apon days..drove me crazy out everyday shaking the biscut tin... shouting, Even Meowing..
nope i thought i lost them mother and Daughter..
then a found out that there was an old woman in the next street was taken every tom,cheeky, harry in..
even my cats!.. i wonderd how there colours kept going missing the old lady was taken the new collors of my cats to give to the other cats...
...
I would make posters up put them through doors , and ask anyone...Did she have a collar or anything on??
Oh the old lady still has one of my cats her new name is Bell...