Sounds like your doing all the right stuff Gill, just keep an eye on her tummy with the milk and if anything just give water. Thing is she will just be glad to have something, anything!!
Does sound very like she could be dumped. A notice in the shop and your car sound like a plan, also the vets and then have a look at the lost and found tips on here cos there are lots of websites listed. Also the local newspaper like someone else said. Lamposts, phone boxes, community notice boards...places where people will see. I hope you have some plastic wallets and sticky tape.
Kittens are pretty savvy so don't worry too much about a room being kitten proof...my two are in the shed which isn't empty but I have put away any sharp things but there is still stuff they can knock over or chew but they are not stupid and don't put themselves in any danger. When I had Soap as a kitten in Jan she just stayed in the spare room, I didn't do anything special in there and she was fine too *hug*
Sounds like you might have number 5 lol!!!