Are you taking into consideration that puppies only need be walked to the thumb rules of 5 mins per month of age, the puppy may well be getting it's appropriate walking times. I'm sure you are not watching the house every minute.

Also not too sure how I would feel about a complete stranger knocking on my door asking if they could take my dog for a walk, I would be more inclined to thinking they were hoping to steal my dog. i would also find it strange to have a stranger invite me to walk with their dogs, infact not quite sure what I would think, everything from he/she is hitting on me onwards?
People often jump to wrong conclusions about things as they don't know others working hours or routines, the puppy may even have a bad leg or medical problem. Or it may not be being walked at all, who knows.................
If you are really concerned then by all means when you bump into this person just make conversation at what a lovely pup they have but I would not mention walking it at all, unless throughout the conversation it is obvious the owner would like help or has worries themselves about lack of exercise, I certainly would not knock on someones door with such strange requests though.
If you really feel there is some sort of neglect then there is always the RSPCA, but be warned there may not even be a problem but a lot of tittle tattle whispers of others going on.