Don't understand what is wrong with 'looked' here.
His Iron-bound trunk had yet to be padlocked, and Julius Kohn looked around one last time before he slipped the lock into the U-shaped hatch.
Gammarly is only software and does not always understand verb tense shifts within a sentence. Whether the sequence of events is logical depends more on the context than on the verb tenses. But the software cannot understand the context. So whenever it encounters a sentence such as yours, it asks the writer to double check. If the sequence is logical, as is your sentence, then the Grammarly flag can be ignored.
You do have a mistake. Iron-bound should not be capitalized. -- it is a compound adjective that modifies trunk and not a proper noun.
|link comment||edited Aug 01 '12 at 02:08 Jeff Pribyl Grammarly Fellow|