Sorry to hear about the tumor.
I don't have any empirical proof, but I suspect it's possible that sickly dogs can send out a 'please put me out of my misery' vibe based on a very strange encounter between our dogs and a neighborhood dog.
We've got two retired racing greyhounds--one is friendly as can be and has never met a stranger (dog or human), the other one is suspicious of everyone and openly dislikes most men, tall people, black people, and people in hats until she gets to know and trust them (which can take minutes or years) and she isn't always interested in other dogs. Anyway, we don't have a fenced in yard (nor does anyone else in our subdivision), so that means we take a lot of walks around the neighborhood for poops and pees.
We know all of the other dogs and dog owners, and our dogs know all the other neighborhood dogs (although we try to stay away from the spastic little yippers, as our shy one has a pretty high prey drive and I don't want to take any chances because it would be over before it would start...) One of the dogs we know is a very old (15? 16? 17? Somewhere in that range, I can't remember exactly, but it's shockingly old) Siberian Husky. It's very slow moving, probably because it's ancient and always near boiling in the Miami heat and humidity.
Our dogs got along just fine with him when we'd stop to do the buttsniff ritual whenever we'd cross paths on walks, so neither I nor the Husky's owner did anything to prevent our dogs from greeting one another. One day, our friendly dog saw the Husky coming down the sidewalk, and when it was about 100 feet away or so, she simply laid down in the grass in the Sphinx position, eyes totally fixated on the Husky. Our high strung dog was consumed with sniffing and investigating, and paid no attention to the Husky at any point. As the Husky grew closer, my friendly dog just sat there calmly and quietly in the grass until the Husky was about 10 feet away and she just absolutely lunged at the Husky, hackles up, teeth bared, jaw snapping and trying like hell to drag me and the other dog close enough to attack the Husky.
Luckily, I had the leashes wrapped around my wrist (a must with greyhounds) and all that resulted was a sore shoulder and a couple of surprised owners. I've never seen my dog behave that way before or since, and the weirdest thing is that the Husky made no move to avoid my dog or to defend itself. The Husky is still alive and kicking, but the owner crosses the street every time she sees us. There are no hard feelings (I was amazed at the owner's nonchalance immediately after the episode 'Eh, they're just dogs being dogs...') and while we've kept the dogs away from each other since then, I don't get the feeling that the dogs have any sort of lingering grudge either.
If I had to guess, I'd say that the Husky put out a signal asking another dog to please put it out of its misery. My dog that snapped is a very fit alpha female, and the one who paid no mind to any of it is a decidedly non-alpha female. The fact that my headstrong and sassy dog was the aggressor and my subservient and suspicious dog was uninterested in any of it suggests to me that it very well may have been some sort of dog hierarchy thing going on. I don't know whether regular interaction is required for any pack dynamics to emerge (we had no interaction with the Husky beyond buttsniffing on walks), or if it's something that's always lingering at the forefront of every dog interaction.
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As far as stoned dogs, I knew a dog who would follow along one person behind the rotation and expect to have everyone blow into his ears.
