Take a look at the sentence again:
Mark looked under the bed,
in the hamper, and
he even searched his brother’s closet, but
he could not find his favorite University of Florida T-shirt.
When you list grammatical items, you must use parallel structure. Here, under the bed and in the hamper are both prepositional phrases, but he even searched ... is a main clause, wrecking the parallelism.
To fix the sentence, change he even searched his brother's closet to in his brother's closet.