Sandeep Kumar's body with several stab wounds was recovered inside the car near a park in west Delhi's Bawana area.
"Prima facie it seems that the constable was killed following some personal enmity. We have registered a case of murder and started the probe from all possible angles," Deputy Commissioner of Police Vikramjeet Singh told IANS.
Sandeep, who was posted with the Supreme Court security but was absent from duty for the past few days, was not in police uniform when his body was found in the driver's seat.
A resident of Kanjhawala in west Delhi, Sandeep is survived by his mother, who has gone to Nashik in Maharashtra to meet some relatives.
Police said the car in which Sandeep's body was found was registered in the name of one of his friends, who is being questioned in connection with the incident.
Officials close to the investigation said someone might have placed Sandeep's body in the car after stabbing him.
"Sandeep joined Delhi Police in 2000. His father, who died 15 years ago, had also worked with the police force," said the official.