In August 1998, Marlon Santos was placed in a foster home in Worcester, Massachusetts with Jose M. Castillo and Yolanda I. Castillo. Marlon's biological mother was homeless and struggled with drug addiction, leaving him with others for extended periods of time. Marlon was last seen in the foster home on November 5, 1998, but his foster parents did not report him missing for two days.

Yolanda claimed that on the day Marlon disappeared, her husband was on a day trip to New York, and she left Marlon and two other foster children alone in the house between 1:00 and 2:00 p.m. She stated that she drove to pick up her two biological children from a nearby school and returned to find Marlon missing. She later drove to New York to pick up her husband before reporting the baby's disappearance to the police.

The Castillos were both ministers and had been foster parents to 51 children since 1993. Prior to Marlon's disappearance, multiple reports of neglect and abuse were made against them, but none were substantiated. Both Jose and Yolanda have refused to cooperate with the police in Marlon's disappearance.

In 1999, Jose was charged with sexually assaulting three female children, one of them his stepdaughter and the other two his former foster children. Authorities stated that the sex abuse allegations came to light during the investigation of Marlon's disappearance. In 2000, he was convicted of assaulting one of his former foster daughters and sentenced to five to seven and a half years in prison.

Police received information in October 1999 that Marlon's remains were buried on the banks of the Wachusett Reservoir off Route 70 in Boylston, Massachusetts. Although torn baby clothing, a diaper, and some pieces of plastic were found in the area, no human remains were located. Authorities believe that the baby's body may have been wrapped in the plastic.

Despite rumors that Marlon may have been sold in New York, the police are investigating his disappearance as a possible homicide. Marlon's biological parents have been ruled out as suspects in his case, which remains unsolved.