Tower Controller reported clearing an air carrier for takeoff but subsequently cancelled the takeoff clearance as the ASDE alarm alert had gone off. An unknown vehicle that was not in communication with the Tower had entered the runway.

Narrative

I was working all the Local Control positions in a south flow. Aircraft X was cleared for takeoff Runway XXR after I had scanned the runway to ensure it was clear. There was a vehicle maneuvering just south of [Taxiway] XX that I was not talking to. I then cleared Aircraft Y for takeoff on Runway XY. The Ground Controller informed me that the pilot seemed to have difficulties with instructions. I had a law enforcement helicopter maneuvering near the departure course of the Runway XY departure. I then restricted the helicopter more east of the Runway XY departure course.I was issuing traffic to the Runway XY departure and getting a readback as I heard the ASDE alarm go off; showing a target on Runway XXR near [Taxiway] XX with Aircraft X departing Runway XXR. The vehicle had entered the runway at [Taxiway] XX. I cancelled the takeoff clearance of Aircraft X near [Taxiway] XY and he began to abort the takeoff. I then told the unknown vehicle to exit the runway with no response from the vehicle. Aircraft X was able to stop the aircraft north of vehicle. Ground Control was able to get Airport Operations out to the runway to check on the unknown vehicle. We were told the vehicle operator had just begun to work the overnight shifts and had a medical issue.Our ASDE equipment has not always been reliable lately. I was glad it was working properly today. I would recommend that we keep the equipment in working order and well maintained in a timely fashion. It may be a good idea to have a 2-person crew operating a vehicle for the first few shifts of an overnight crew to ensure they are aware of the surroundings at night.