Take a look at the sentence again:
Celine looked behind the toilet,
in the laundry basket, and
checked under the bed, but she could not
find Squeeze, her nine-foot albino python.
When you list grammatical items, you must use parallel structure. Here, behind the toilet and in the laundry basket are both prepositional phrases, but checked under the bed begins with a simple past tense verb, wrecking the parallelism.
To fix the sentence, change checked under the bed to under the bed.