Two ambulances raced to Hayden Panettiere's death scene and rolled out stretchers, which seems to confirm what we've been hearing ... that Hayden wasn't the only one in the apartment unit.

Law enforcement sources tell TMZ ... the 2 ambulances were dispatched to the scene, although no one was wheeled out of the apartment and the ambulances left without a patient inside.

A source tells us, Hayden's on-and-off-again boyfriend, Brian Hickerson, was living in the unit with his brother. Hayden and Brian flew from L.A. to South Carolina Saturday and were both at the apartment.

We have not been able to confirm if Brian was in some sort of distress, such that the caller would report 2 people in need of an ambulance.

As we reported, Hayden was pronounced dead at the scene and her body was taken away in a coroner's van.

According to dispatch audio, Hayden was in cardiac arrest and the caller suspected it was the result of an overdose.