The alleged abuse of a client of the O.D. Heck Developmental Center came to light only because two private investigators happened to be eating lunch one parking space away outside the Park Avenue McDonald’s, according to a state investigator.

On the road for an unrelated case on behalf of a Tennessee-based investigative firm, the two Vermont-based investigators were sitting in their car just after 12:30 p.m. June 5 when one of them said she saw a caregiver punch what appeared to be a client inside a state van, Antoine Karam, an investigator in the state attorney general’s office testified Tuesday in City Court.

Pamela Russell reported the sighting to her boss, retired Vermont state trooper Robert Duhaime, Karam said.

Duhaime in turn reached out to Karam, who he knew through his son.

Russell’s claims and Karam’s preliminary investigation, later turned over to city police, formed the basis for the arrests of four O.D. Heck workers, who all face one misdemeanor count of endangering the welfare of an incompetent or physically disabled person.

The Niskayuna facility is run by the state Office of Mental Retardation and Developmental Disabilities.

One of the four, Christina Brandon of Troy, 47, initially also faced a misdemeanor assault charge for allegedly punching the adult female client, who is described as profoundly disabled and only minimally able to communicate.

According to police, Brandon was angered when the client, who was part of an outing to a horse farm, spilled her salad on the floor of the van when the group stopped in the city for lunch.

City Court Judge Joseph Sheehan dismissed the count in October.

The other three defendants — Dranae Washington, 37, of Schenectady; Natalie Richardson, 49, of Schenectady; and Sharon Butler, 49, of Schenectady — were charged for allegedly failing to assist the client and report the incident. All four defendants were placed on leave after the incident.

Defense attorneys for the women questioned Karam about how well Russell could have seen what she described.

Karam acknowledged under questioning that Russell’s brother-in-law, who was in the car with her, did not see the alleged punch because of where he was seated. Karam said the brother-in-law did capture a brief video clip that police used to identify Brandon. The video did not record the alleged punch.

The hearing is scheduled to resume next week.