From the description you give it sound abandoned, but may be stolen. Go and get it, put it in your shed then ring the local police and file a found report. Give them an accurate description including frame number and tell them you'll look after it until they find an owner. The police DO NOT WANT IT CLUTTERING UP THEIR SCANT STORAGE SPACE. Many less scrupulous people find hedges a more suitable and convenient last resting place than the local Household waste Depot, and you'd be surprised how many people think a rusting £50 tesco wonder is a decent bike, so we tend to get literally hundreds of abandoned heaps of scrap brought in by Gordon Goodcitizen.
You have then shown that you are acting honestly, and not stealing the bike. If it remains unclaimed after a month or so you can then do with it as you wish, with one small caveat, and that is that the original owner never loses claim to their property. In the unlikely event that you restore the bike to concourse condition and the original owner comes along, you would then have to (theoretically) give back what was once theirs.