Kittens Missing

We have a stray cat at our home...she usually comes by for food. Recently, she had five kittens whom she brought to our home. We may have had them for maybe 3-4 weeks.
From, yesterday the kittens are missing, the cat seems to be searching for them as well... She seems like she is crying-she lets out whimpering noises.
Nobody couldn't have taken them from our home and the kittens aren't old enough to jump to the walls.. We have searched everywhere but to no avail.
Any idea where they could be?