If it were me---and it has been many times---I'd take the dog to a shelter the next time it's running the streets. Hopefully, a no-kill shelter.

Talking to the owners is pointless. If they care at all, they will look for their dog and retreive it from the shelter. The shelter will discuss it with them. If they don't retreive the dog, the dog can find new owners.

If you don't have a no-kill shelter near you, alert a lab rescue now so they will be prepared if you need to take this dog to them.

Mary