Air carrier Flight Attendant reported the gate agent attempted to open an armed door without knocking first. The door was then disarmed and opened with no escape slide deployment.

Narrative

Note: Due to a rejected takeoff; we were required to wait approximately an hour to allow the breaks and tires to cool after the initial takeoff attempt.Following the rejection of the takeoff; we returned to a hardstand for maintenance purposes. After the initial maintenance issue was resolved; the aircraft door was closed once again.The cabin was prepared for departure. Within minutes of the door closure and cabin preparation; the Captain announced that the maintenance issue persisted. In his announcement; he stated that the maintenance team was investigating the problem from the ground and that he would turn off the seatbelt sign for the passengers.The Captain never provided me with instructions to disarm the doors; as the maintenance team was not expected to board the aircraft.At this point; I was located in the mid-galley area. When the seatbelt sign was deactivated; one of the flight attendants made an announcement to disarm the doors. I immediately approached the Captain and inquired if he wanted the doors to remain armed. The Captain responded that he wanted the doors to remain armed for potential emergency reasons. I then proceeded to make the arm doors/ all-call announcement. We armed the doors and conducted the all-call procedure. After the all-call was completed; I informed the crew of the Captain's instructions regarding the doors remaining armed and instructed everyone to remain by their armed doors.Minutes later; I was in the flight deck discussing the delay with the Captain when I heard someone yelling my name from within the cabin. Myself and the Captain exited the flight deck to investigate the commotion. Flight Attendant A was stated; They attempted to open the door." At this point; I observed Flight Attendant B looking down the aisle (from door 2L) and appeared visibly panicked. I made the disarm announcement and conducted the all-call procedure once more.I proceeded to door 2L to observe the situation and noticed that the air stairs were in place and an agent was at the door. Flight Attendant B informed me that the agent had attempted to open the door without knocking. I stood at the door; awaiting the agent's knock. The agent knocked and peered inside; but she appeared to be experiencing difficulties in distinguishing my thumbs up gesture (I verified the door was disarmed and then made the gesture). After a moment; she finally opened the door and promptly apologized for attempting to open it. She explained; that it was hard for her to see inside.Captain informed us that the door would automatically disarm itself if opened from the outside."