Well, I stopped by neighbor's house a couple days after this posting and the she did say that the dog bag there was mine and to please pick it up.
Me: "It's not mine. I can tell by the newspaper bag. I don't use those bags since I don't have a subscription."
Neighbor: "It's is yours. You left it here last time you were here." (which was 6-7 days prior)
Me: "It's not mine. But I'll pick it up this time and put it in the trash." (The full trash barrel was on the curb as it was trash day-eve.)
Neighbor: "No. I don't want poop bags in my trash. You have to take it home."
Me: (refraining from pointing out that she does use her trash barrels for poop bags (her dogs and others')). "I'll bring this bag home this time. But in the future, I can only bringing home my own dog's "calling cards". I'll purchase distinctive bags to avoid the confusion."
She looked surprised but didn't respond.
So yesterday, doggie and I stopped by and fortunately she wasn't home because I burst out laughing at the full yellow newspaper bag on her stairs!
Again not mine. Besides the fact that I don't use those bags, this time I could see the knot which was a regular knot. I always use a loop knot in order to carry the bag by the loop and even reuse the bag if possible. I didn't see the knot on the last bag as I hadn't wanted to touch it since it was so old and yucky. I had just picked it up with another plastic bag inverted over my hand.
It's also occurred to me to start saying every time I leave her place to ask "did I have a bag with me? do you see my bag." then she will check to be sure I have my bag.
So, of course, now I'm curious - who is the person leaving the yellow newspaper bags?? I'll be keeping my eyes open...