A similar thing that happened to me in my grief. I was soooo overwhelmed with how Kashka's euthanasia went (I had the sense that she did not want to go at that moment - I have reasons for thinking this which I wont get into here). The morning after I put her in the ground, I heard the saddest cat cry coming from the direction of the woods in the back yard - so defeated sounding, my first thought was -what if ts Kashka, her spirit communicating with me. Not that I go around believing in things like that, but I dont disbelieve either - anything's possible and I think I was just so distraught, and overwhelmed by grief, I just latched on to that as a possibility...for lack of any other explanation.
I also thought it might beone of my cats so I ran downstairs, into the backyard in the direction I thought I heard it coming from, but as I was heading that way, I heard the sound again, but from behind me, in another direction entirely, and far away sounding, could barely hear it. It wasnt my cats because they were lying on the picnic table sunning themselves.
This just sent me around the bend - I had been starting to accept her death, but hearing this distressed cat sound that had no explanation just reactivated it big time. I couldnt really say it was Kashka communicating with me from beyond the grave - but thing is, I couldnt really say it WASNT her either.
So a few weeks passed. I happened to be sitting in my backyard when I heard the same sad sounding cat cry. This time I identified it - it was my neighbor's cat up on a second floor porch, stuck outside and wanting in! A huge weight lifted- I think that feelings of guilt and grief just have a way of coloring every situation. How you interpret the experience is filtered through the emotion. Had you heard that at another time you would have probably come up with another explanation - neighbor cat, etc.
Also please note that second guessing and doubting yourself and what you thought was going on at the time seems to be a common feature of grief, part of the process. A lot of us just havent had a lot of direct experience with death, so when it happens - its confusing and upsetting.
I wonder if for your own sense of closure it would make sense to ask your neighbor about what happened, has the cat been buried and letting him know how much it means to you that this cat was treated respectfully in death. You might not even have to come right out and ask the question (was the cat really dead) but maybe just by the things he says and how he talks to you about it might give you some peace around what happened. Do you know him very well? It sounds like a kind offer.
Take care, cathy