Another thought came to mind. Perhaps this dog is a totem animal, yours, or your hubs, or one of the kids? Seeing you have noticed it most, if it is a totem animal, I would lean towards it being yours. Just a thought. Maybe he/she is a member of Chester's family, a parent or some member in his blood-line, that is checking in on him? With his not able to see well anymore (aww sorry to hear that hugs to Chester), a spirit animal might be there supporting him? Why wouldn't/shouldn't/couldn't animals have spirit visits as we do?
You noted Chester has noticed/followed something before that you couldn't see, if he wasn't agitated by it, he very well may have recoginized the dog if that is what he was seeing? =)
Keep a camera handy, maybe you will luck out and snag a photo of the dog, that would be really cool. Who knows, it's a long shot, but maybe something will be noted in the photo which may indicate there is another spirit visitor with the pup, one of your loved ones perhaps?
![[Rainbow Smiley]](graemlins/color.gif)
Any other unusual things happen before or since you have seen this dog? Even something small you thought odd but just shrugged off at the time? Spirits will give us little signs which can easily be missed and/or dismissed. IMO Small items being moved (picture for example), keys or jewlery going missing for a bit, light flickers (which can't be explained), things with the phone, feeling a light touch, tv turns itself on, etc etc. Just something to consider.
Perhaps some others may have some thoughts/opinions as to what this might be, or they have experienced something similar before? Do keep us posted if anything else unusual occurs or if you see the dog again. =)