The man dressed in gray is Eddie Mars. Marlowe stalls for time by saying that he and Carmen are business acquaintances who came to pick up a book from Geiger.
Mars doesn't buy Marlowe's story. He lets Carmen split, but makes Marlowe hang around a while longer
When Mars notices the blood stain on the floor, Marlowe claims he hadn't seen it and explains that he's a private dick, hired by the Sternwoods to look into a case of blackmail.
When Marlowe asks why Mars has a key to Geiger's place, Mars says that he owns the house and that Geiger is his tenant.
The conversation between Mars and Marlowe is fast-paced and hardboiled (see "Writing Style"). Mars claims that he only wants to know Geiger's whereabouts, but Marlowe tries to gauge Mars' reactions by saying that he knows about Geiger's pornography racket.
Eventually, Mars becomes annoyed with Marlowe's attempt to avoid his questioning, so he calls in his two bodyguards, who frisk Marlowe for weapons. The bodyguards also confirm that Marlowe has a detective license and they finally let Marlowe leave. Phew, that was close.