Air carrier Flight Attendant reported a gate agent opened an armed door without notifying the crew prior. The escape slide did not deploy.

Narrative

After all passengers boarded; the Gate Agent (GA) came onboard and said boarding was complete but she was going to check on one passenger. At that time; I played Boarding Complete and turned to talk with the flight deck to tell them that the GA was onboard; but other than that; boarding was complete. I no longer saw the GA onboard when I turned around; but rather than assume she got off the plane; I called the FA B to verify that the GA deplaned and that we were good to close the doors; i.e. that we were in compliance in B. The B-FA confirmed that the GA exited the aircraft and we were good to go. I then asked the CA for permission to close the boarding door; and he said yes. The GA was not present as I closed the door. Next; I began to gather all the pre-departure drinks. As I was putting the last one in the trash; the GA was opening the armed 1L door. She did not knock on the door or call the CA to let us know she was going to do this. I was relieved when the slide did not deploy! At this time; the GA pulled the temperature probe out from underneath my demo kit; and told me I was supposed to always check this before closing the boarding door. I agreed and apologized. After she retrieved the probe; I popped my head into the flight deck (the door was not closed yet because I had been gathering the pre-departure drinks). I communicated with them what happened and told them I re--armed the 1L and that we were secure for taxi.