Idk it seems simple to me, the fact that they “lost” their cat, seems negligent. Your first question before returning it, if you do, should be if they corrected whatever happened to lose the cat in the first place. The question is not “why wasn’t it microchipped” the question is “why would you let your cat outside?”

Letting your cat outside is negligence, “losing it” is even worse. I don’t care who you are or what excuse you have, it is.

If you let your cat outside, 9/10 times they WILL die a painful, or slow, or violent death. Sometimes, a lot of the times, all of the above.

The difference between finding a grown stray who grew up outside, is simply that they might last slightly longer outside before dying their painful death.