Sif might not have the honed senses Matt does that allow him to react so quickly to his environment, but she's been a warrior for a very long time. She doesn't sense the approaching attackers so much as she feels in her gut that something is approaching, and Daredevil's warning confirms her instinct.

Her blades flash again, spinning in a wide arc to catch whatever it was that might be foolish enough to catch her unawares, and though they meet resistance, she sees nothing.

Wonderful--an invisible attacker. "I can't see them!" she calls back to Daredevil, her voice more annoyed than concerned.