Ummm... Yes, it is (or might be) stolen. Something 'abandoned`is almost certainly likely still to have an owner. As far as I recall, the leading case in the UK was a dead pig, buried by its owner because it had died of disease, but dug up and sold. Once the taxi company has hoist this in board, they might be a bit more eager too help. If they say they can't trace the driver, try Health and Safety at Work, employment law, anything you can think of. Got any chums who are practising lawyers? Odds are that one 'phone call would put the fear of God into the taxi firm.
Apart from that, commiserations and good luck.
Cheers,
R.