I found a stray cat who was an absolute sweetheart, and took him to the vet. He was microchipped. The vet staff and I spent several days tracking down his owners, who had moved several times and had a new phone number, but we eventually found them and reunited them with their kitty. As it turned out, he'd been missing for several YEARS! I was amazed that no one else had had him scanned, or maybe no one had been as persistent as I had been in finding his owners.
He hadn't had a collar on when I found him, but for the few days I was feeding him before I could get him to the vet, I put on one of my cat's spare collars with one of those ID tubes on it, with a piece of paper saying "If you are the owner of this cat, please call (my phone number) and let me know so I know he is not a stray."
As for my own 17-year-old granny, she is both chipped AND collared. I have the vet check the microchip every year, to make sure it hasn't moved/migrated in her body.
Her collar has a tag with her microchip number on it, as well as two separate types of ID tag (the regular hang-tag kind, and one of those flat ones that fits directly on the collar itself). I also have a teeny-tiny short-range GPS tag on the collar, since she's an old girl and has medical issues. Luckily I've never had to use the GPS!
But I am a firm believer in microchipping AND collaring cats, even "strictly indoor" cats. My friend's cousin did not have her Scottish Fold spayed, collared, OR chipped because "she is an indoor cat that will never get out" -- of course someone left a door open and the cat DID get out. Luckily they found her again, but she was pregnant. My friend herself has the same attitude towards her two cats - no collars or chips because they "will never get out" - luckily both of her cats are neutered, but her brother accidentally broke a screened window one day and both cats dashed out of it before anyone could grab them. Again, luckily, my friend quickly found both cats, and nowadays both sport collars and ID tags ;}
(unfortunately I think her cousin never listened to the advice, and although she kept both kittens her cat gave birth to and had all three spayed/neutered finally, I don't think any of them are chipped or collared. sigh.)
Obviously I feel very strongly about microchipping and IDing kitties, and there are so many safe breakaway collars out there that "oh he/she will strangle himself!" is not really, imo, a valid excuse for not collaring a cat, whether indoors or not.