God bless you Grumpy for helping doggy. By the sounds of things, it's hopeful that the owners will be frantically looking for it...it's lasted this long with them, so here's hoping.
But you are making a huge difference to the dog's chances of survival and being returned to it's owners, well done you.
We recently found a dog wandering on a very busy road, I pulled her off the road before she was creased by a car. We went to great lengths to find her owners that night, and we did find them. We never met them but their neighbours kids took the dog in and it was so rewarding to see the dog get out of our car so happily, secure and relieved that she was home. She had been wandering for a couple of days and was a very sweet dog, quite old. I dread to think what would have become of her if we hadn't made the effort.
Keep up the good work. You could try ringing the guards just to ask them to make a record of it so if the owner happens to ring, they would have your number. Best of luck with it...it will be worth it in the end.