What did she think she saw

So we’re on our daily walk when Blondie suddenly goes into full alert status, staring straight ahead, straining to keep something in sight.  Only she’s the only one who seems to see it.  She continues on full alert as we approach the spot where the invisible seems to be. We get to the spot. She stands there a moment. Looks around.  Then casually wanders off.  And I have to wonder what she saw…what she thought she saw…whether she has finally lost her grip on that small portion of her mind that wasn’t addled already.