I lost a 7 month old cat/kitten two years ago this way.

But I don't know "who" has her...our Vet says someone has her and it is common.

I went door to door, with a photo, but no one admitted to having her. I did get many who said they "saw" her. We did everything to find her though. She was chipped too and neutered. She learned "outside" like all our other cats. We have a cat door (now locked.) She was from a feral home, but she didn't act feral, but we thought she might have been engaged by a male we sometimes see. None of the dogcatchers or rescues here found her dead or alive--they all wand the animals, and our kitty was chipped and they would have called us immediately they all said.

I staked out the area finally for a week in my car at dusk and sometimes at night where most of the sightings were. I put out tuna bait too...but only saw the male.
Either these people were lying to me, or really saw her??
Our cat was a beautiful tuxedo type, and very distinctive in appearance.


So if someone did take her, they made her into an indoor cat.

I have two ideas on the matter.
1) stake out her home in your car and watch at night to see if they come out. Take a camera (mine takes photos in the dark).
Then you have proof that they are there. (this won't work if the neighbor converted them to inside only, obviously.

2) If you know any neighbor who might be willingly to help you, you could finagle secretly that this other person could get invited to coffee or inside socially where they could SEE the cats, then keep mum and report back to you that they are in there, and you go armed to the police. (your "spy" might take a phone photo if possible while on the inside). You have your chipping proof that the cats belong to you and with any further evidence the police should be able to get them back for you.
I'd suggest not letting them out ever again if you get them back, or replaced with a new one.

In our suburban neighborhood, in SE Michigan we have now had sightings of a cougar---yes cougar--- and several reports to police about coyotes. These coyotes are everywhere it seems, now and even den in suburban bushes! , and Nat Geo TV channel has had documentaries about this. One took place in Chicago. In downtown Detroit there was an incursion not too long ago of a coyote in a snack store near the RenCen building complex! So it is possible that your kitties met with this fate.

I know how heartbreaking this is for you. It sadly brings it all back to me now.

We have two kitties now...the older one was used to some outside time before we lost the other. I allow her outside during the day, only, where she sticks around our home (she is a scaredy cat and very timid. At dusk she comes to be let in for the night or hubby rounds her up for dinner). The other younger one is 2 yrs old now, and never goes out. Only on our screened in porch. It was difficult for us to move to indoor only cats, but I won't go thru what we did for our lost one, ever again! When we live on our UP island, we let Weezie out...day only and she comes in willingly at dusk... she is a bit afraid up there, but loves to catch the little pigmy shrews. I see a day coming that she won't go out anymore at all. And that is now fine with me.