Hello,
Yes, mother cats will leave their kittens for short periods of time to hunt and/or feed. They really need a lot of extra nutrition right now so I recommend feeding a high quality kitten food like Iams or Hill's Science Diet. Those will give her the energy she needs during lactation and nursing.
As to If she had the kittens, that can sometimes be hard to tell for the first few hours. Check to see if milk comes out of her nipples when squeezed...if so, she is either within a day or two of having them or already has had them.
See if the hair around the nipples is wet or disturbed and look for traces of blood around the vaginal area also.
If you are considering keeping this stray, the first thing you should do is have her tested for Feline Leukemia (FeLV) which is a deadly disease that can easily be spread to other felines.
If she tests positive, it is most likely the kittens have it as well (don't let her near any cats you may already own). Unfortunately, there is no cure for FeLV but there is a vaccine which helps prevent them from contracting it.
FeLV is highly prevelent in stray breeding cats since it is mainly transmitted through breeding and fighting (exchange of blood and/or saliva).
I hope this information helps and good luck with the kittens if you find them! Those momma cats are GOOD at finding the perfect hiding place!
Regards,
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