The incident is now under investigation and has reignited concerns about those charged with the safety and security of riders.
The guard in question is employed by Allied Universal, which SEPTA has contracted to serve as a supplement to police who patrol the transit system.
In a statement to Action News, SEPTA spokesperson Andrew Busch said in part, "an investigation into the incident on Saturday at the Allegheny Station is ongoing."
"According to preliminary information, an unidentified individual stole a backpack that belonged to an Allied guard who was assigned to that location," continued the statement.
SEE ALSO: Action News investigation into SEPTA's safety record reveals alarming trend
SEPTA officials said the unarmed guards, mainly stationed along the busy Market-Frankford Line, were hired as an added presence and are only meant to observe and report issues to SEPTA police.
Just recently, an Allied guard was arrested and now faces assault charges for an attack also caught on video this past July.
In that video, the guard is seen lunging towards a man before pummeling him with punches.
RELATED: Video shows security guard punching man at SEPTA's Arrott Transportation Center
"It sounds good in theory to have them observe and report, but the reality of it is it can go beyond that and we just don't think they're equipped to handle what they are asking them to handle," said Fraternal Order of Transit Police Vice President Troy Parham.
Parham said the union has called for the end of the security guard program in light of these attacks and other incidents that have not yet been made public.
Parham also said the union would instead like to see more officers hired, which would also help with crumbling morale.
"We're extremely thin. We've lost, like, almost 30 officers in the past seven months, so we want to see an increase in police personnel so we can do our job," he added.
Union officials said they are ready to meet with SEPTA leadership to find a solution.
In a statement, Allied Universal said:
"We are aware of an incident that occurred at the SEPTA Allegheny Station in Philadelphia with one of our security professionals. Allied Universal takes all reports of employee misconduct very seriously. Such incidents do not reflect our company values, culture and high standards we set for our security professionals. This employee has been suspended pending an investigation. Since this is an active investigation, we are unable to provide further information at this time."