Colored Stickers Target Homes of Dog Owners?

Warnings about dog-snatchers tagging homes with colored stickers to facilitate the stealing of canines for use in dogfighting events were originally circulated in the suburbs of Perth, Australia, in February 2013 before being relocated to a UK setting the following month:

Any dog owners beware, leaflet droppers or anyone walking around housing estates.
Keep your eye out for small coloured STICKERS on gates or doors, gangs are marking how many dogs live there to steal and they are using them for DOG BAIT FOR FIGHTING.
Ring the police immediately if you see any and inform the home owners.
Remove the stickers immediately!
Red stickers are for big dogs
Yellow for medium and
Pink for small breeds
Please re-share and keep your dog safe!
This is already happening in the Goole Yorkshire area, UK and could be spreading across the country. This is disgusting

The same rumor has also been spread involving plastic bags in trees:

I've seen on Facebook that people's homes are being marked by plastic bags tied in trees indicating they have a dog. This is a sign for criminals to abduct the dog for it be used as bait in dog fighting rings. This just doesn't seem feasible.