yeah you probably could, if you moved it, used it, broke in... that would be considered stealing it in no uncertain terms i'd expect
but if you were just stopping its degradation vandalism or removal as abandoned, for the true owner.....
who may not return... maybe the law would be kinder
as in the case of the one i lost....
VW Karmann Ghia
left in carpack at little Venice, Paddington, by member of US military who was posted to Iraq
he never came back....no indication his posting was fatal but none to indicate otherwise.
his flat was eventually cleared.
My mate Simon worked for the parks n gardens, knew i had a VW fetish at the time, kept an eye on it for 18 months until one night the kids stamped the roof in and smashed the screen.
we covered it with poly and tarps, we found the tax disk on the floor had been un-taxed for 6 months, I called DVLA and council.
DVLA very helpful. if its not taxed sir you can tax it and then provided you are willing to pay any fines tickets etc you can become registered keeper
But the car will not belong to you. you risk financial loss and action from any finance company from the council or police or from the owner who may assume you damaged it.
if they don't claim it in 5 years you can apply to be owner
it would be illegal to do anything to it before you have paperwork in hand.
but i'd called the council.... who came the next day and slapped a big sticker on it. towed it to a breakers yard on a bridge over the railway in Hendon where it was stripped and the body crushed.
it shared a yard for about 3 months with everything that had lost its owner, left on street, and clearance from council property...quite a range of vintage tin
it just seemed like such a waste abandoned/untraceable owner and its treated as insurance write off Cat A or B....madness.
however this was 15 -20 years ago. things change you may end up in prison
Dave