Take a look at the passage again:
Marina, the beautiful mermaid, wanted some
tuna salad. But had a small problem since she was allergic to celery. At
Sammy’s Sub Shop, Marina hoped to find tuna salad free of this dangerous
vegetable. Flopping across the tiled floor to the counter. Marina placed
her order and then checked her sandwich for celery.
Not noticing, however, the spoiled mayonnaise.
At five o'clock that evening, Marina became
violently ill with food poisoning. When a lifeguard at the beach discovered
the problem, he called 911. Even though the mermaid had fishy breath. A
handsome paramedic gave her mouth-to-mouth resuscitation. Wailing like a
sick dog, the ambulance sped off to the hospital. Where the doctor on call
refused to treat a sea creature with a scaly tail. A kind nurse, however,
had more sympathy. After she found some Pepto-Bismol. Marina drank the entire
bottle of pink liquid, feeling an immediate improvement. The mermaid told
the rude doctor never to swim in the ocean. For she would order hungry sharks
to bite off the doctor's legs. While sharp-clawed crabs plucked out his
eyes. Tossing her long hair, Marina thanked the nurse for the Pepto-Bismol.
And took a mint from David, the handsome paramedic.
A sentence must have a main clause. A main clause will follow this pattern:
Subject + verb = complete thought.
In the highlighted item, noticing is the word expressing action. The ing that ends it, however, makes noticing a participle, not a verb. Without a verb, you cannot have a main clause.
All you have here is a participle phrase fragment. Better luck on the next one!