I was walking back to my house through the parking lot, having just taken out the trash, when i heard the sound of large helicopters arriving - then i saw two helicopters flying in loose formation (about 2000 ft. apart at the same altitude (approx. 2500 ft.) and one trailing the other) they both had either bright landing lights, or search lights pointed in the direction of flight. i assumed they were military because we often see formation flights of two of either ch-47 or uh-60 flying along the river (delaware river). however as the flight came to nearly over head, i observed the lead helicopter was actually following another unknown aircraft, which was approximately 1000 ft. in front of the lead helicopter. this unknown aircraft did not have flashing anti-collision lights ("strobes") nor the primary red flashing light, however, it appeared to have one or more very dim whitish lights, not flashing. as this unknown made a gentle turn to the south, the flight of two helicopters pursued this unknown. since it was after dark, i was unable to definitively identify the type of helicopters these were, but i could barely make out the size of the airframe against the back-drop of the night sky - on the other hand, i was not able to make out any definite airframe size or shape of the unknown these appeared to be pursuing, except for very dim unflashing light(s). it looked as if they were pursuing a small drone, except the airspeed of the flight was doing at least 90 knots or faster, so i don't think it really was a drone. there was no sound of a different engine type other then the two helicopters. the flight of the two helicopters perusing the unknown disappeared to the sw of my position at a level flight altitude of about 2000 to 2500 feet.