Five months prior to stardate 44356.9, Marc died in an accident. Shortly following the funeral, Janet had all of Marc's belongings removed from their quarters. Unbeknownst to her, Deanna Troi had saved one special item, a music box, just in case Janet changed her mind. The grief over his death led Janet to seek counseling from Troi, where Deanna returned the music box to her. (TNG: "The Loss")
It was never made clear whether or not Marc Brooks was a Starfleet crewmember, although the fact that Troi was present following the funeral and that Captain Picard seemed familiar with Brooks implied he was aboard the Enterprise at the time of his death.