hello, you should capture it and take it to the vet to see if its chipped, even if its owners are close by it might shock them into keepign an eye on thier kittie some more, really at its age i would still only be letting it out under supervision for small intervals. If its a stray then you need to get it into rescue asap, especially if there is a tom loitering around, if its female its still too young to come into heat (i think usually around 10 months is average) but you cant be sure its only 5 months, and if its male it could get some nasty injuries from the tom.

Can you put some posters around?
Also white cats are prone to deafness its possible its got out and been spooked too far from home.