LYNN – Police are investigating why a man was found stabbed and beaten Sunday in a pickup truck outside Union Hospital on Lynnfield Street.David Nassor, of New Hampshire, was later transferred to Massachusetts General Hospital in Boston for life-threatening injuries, police said.Hospital security located Nassor lying in the back seat of a pickup truck in the hospital parking lot shortly after 9 p.m., according to police.He was listed in serious condition on Monday evening.Lynn and State Police detectives are investigating the incident.