In this case the prepositional phrase in this sentence is in the door’s keyhole.
Why? Because a prepositional phrase is used as an adjective or adverb the use of a prepositional phrase is to answer the question: Which one?
So you could think of it as in the door’s keyhole as answer to Which one?
The prepositional phrase allows the reader to understand which key is being talked about. Which key? The one in the door’s keyhole!
Hope my explanation helped you out and I used Grammar Bytes! :: The Prepositional Phrase to remind my self what exactly a prepositional phrase was. I advise you to use the link if you want to further knowledge yourself into the idea of prepositional phrases.
Have a good one!