See, while Madoka might not be able to notice what Kitty is doing, she is very much noticing what the sali themselves are doing - if not just for the fact that it's pretty noticable that they are no longer attacking Madoka.

So when she looks over and spots them looking around like that, she gasps.

"Watch out..!" Madoka calls out, though she isn't more specific than that - because she doesn't want to let the sali on to the fact that Kitty is there, or what she's doing. She figures Kitty will know a general warning is meant for her, anyway, since they're the only ones on the same side here.