I am apart of a few local Facebook groups for missing pets so I can share and keep a lookout if I see a post! What I have noticed though is a post regarding a missing dog will get hundreds of shares but when its a cat no one seems to care. I presume that's because cats often roam outside on their own whereas dogs don't.
I can imagine if you have an indoor cat that's not used to being inside this is a major worry, but how will others know that your cat is an indoor cat and not just another outdoor roaming cat? As a safety measure, I would really recommend buying a personalised/engraved ID tag. I managed to get one for as cheap as £3 on ebay, that includes the tag, the engraving and delivery (Bargain) !! On mine its states:
"IF I AM OUT ALONE I AM LOST. PLEASE FIND MY OWNER+NAME + TEL. NO. + ADDRESS"
This way, if your indoor cat goes up to someone they will be able to read the ID tag and know that they aren't supposed to be outside alone! Heres the link for my £3 tag, although there were plenty of other cheap alternatives you just have to look around.