Stray and Indoor Cats

We have two male indoor cats that we only allow outside in the yard in
the evening while we sip a cocktail and keep an eye on them. Lately, a
female stray cat has decided to make our yard her home. After a few
weeks of hanging out, my wife has even begun settting out some food and
water for it. Oddly, our two indoor cats don't seem to mind this cat's
presence, and calmly watch her through the glass door when they usually
make quite a fuss when some cat or other critter visits their domain.

My question is whether there's any risk involved for our indoor cats by
letting this stray cat hang out in our yard? Could the stray pass
along some kitty ailment to our cats, even if they never come in
contact with each other?

-Fleemo