Missing Cat

I'm doing house sitting for 7 days. (This is currently my 2nd day) I'm supposed to be caring for 2 cats and filled up both bowls with water and food as the owner instructed me to do so. The owner has one senior cat and a kitten, but she got the kitten a few months ago.

However, since I've been here, I have NOT seen the kitten. I was even extra careful when I entered the apartment (As I am always with all pet clients just in case!) I have not left the house since and I've been looking everywhere and the windows and doors are even closed. I don't even hear any meowing. The other senior cat I've been seeing more often. The owner does not even have a yard and lives in an apartment. The food bowls also look just about the same and the senior cat has been eating from both bowls of wet food only.

I did inform the owner and she said that it's probably because the kitten thought that she had abandoned her or something. Both cats seem friendly and calm during the Meet & Greet. I'm even providing constant updates to the owner. What should I do?

Edit: As I said in other comments, the owner told me the kitten is most likely in her room. I placed some dry food out and when I got home in the evening, the food was almost gone. But the door was open (I can't close it) The kitten often eats dry food only though.

I also heard meowing just now at nearly 1 AM (While the senior cat was laying down next to me) and I have seen traces of small poops in the litter box