It took me a while to discover their new hiding spot. The sink on the back porch has three cupboards below it. There's a double-doored cupboard under the sink itself, and two single-doored cupboards flanking that one. One of the single-doored cupboards has a door that is easily opened by the cats, and that's the one the kittens had slept in the first few nights after they came onto the porch.
The doors of the other two cupboards shut tight, so I didn't think to look in them at first, but then I recalled that the loose cupboard has a space behind it through which the kittens could enter and exit at the end of the whole assembly. I looked in it and realized that there is a corresponding opening leading to the adjacent cupboard. When I opened the door of the center cupboard, there were the kittens, staring at me. Clever little cats have found a substitute for their hiding place in the basket of the lawn mower, which is no longer sitting in the back yard.
So now I'm feeling both relieved and sad; relieved that the kittens are still here, and sad that they haven't been taken away to get new homes with someone who can let them live indoors and feed them better food than I can afford. These kittens are going to make me sad no matter what. Cruel little beasts.