It's very common for kitties to hide their babies - if you cannot find them, they should be safe from predators. With ferals, we look for signs of nursing - the fur will be pushed away from the nipples which stand out. Her udders are usually pronounced. Are you feeding her often with a good quality kitten food? That will help ensure that she has adequate milk supply.
If your cat is going in and out, then she is nursing her kittens and will likely bring them to you when they are a few weeks old and ready to eat wet cat food. If so, you’ll need to bring everyone inside so the kittens are safe and mom cat can be spayed or she’ll be pregnant again.