Allen was last seen in Philadelphia, Pennsylvania on December 13, 1985. Every day his mother would call home from work at 7:30 a.m. to make sure he and his sister were awake and getting ready for classes at Bartram High School. When she called that morning, no one answered. She called again at 3:30 p.m., an hour after school was out, and still no one answered. Allen's mother was working two jobs at the time and didn't arrive home until 11:30 p.m. She discovered Allen's sister had come home at 11:00 p.m., but she was locked out because only Allen had the house keys, so she had gone to her grandmother's home. Allen has never been heard from again.

There are reports that Allen's school closed early on the day of his disappearance. His mother doesn't believe he went to school at all that day, but police records indicate he was at school at some point during the day, along with a friend. The two of them rode the subway around Philadelphia all day, then came back to their own neighborhood by bus. Allen's friend stated he last saw him exiting the bus at Woodland Avenue, and Allen told him he was going to see a female friend. This is the last confirmed sighting of him.

Authorities don't believe Allen ran away from home. He left all his clothes behind, as well as $50 his mother gave him as a birthday present, and he had not spoken of any plans to leave home. His mother stated his hobbies in 1985 included rap music and basketball, and she described him as a picky eater. Authorities have attempted to re-interview Allen's friends in recent years, but owing to the passage of time many of them cannot be located.

Allen's case may be linked to the disappearance of Christine Green, who was also a student at Bartram High School and disappeared in 1985. Neither case has been solved.