As for stray dogs, please either try to catch them or at least make a mental note of their description and the place you saw them. You never know whether they are just 'latchkey dogs' that have been turned out or whether they are someone's much loved pet either escaped or turned loose after being stolen.
Please visit
http://www.doglost.co.uk and you will see how desperate people are to find their missing/stolen dogs and how such a sighting can often lead to them being reunited.
Also please consider joining Dog Lost as a helper, you will be emailed details if a dog is missing in your area, and you can look out for it, or put up posters if you are able to.
The more people involved the more likely they are to be reunited. Think what would happen if it was your dog that was missing.