Air carrier Flight Attendant reported that a mechanic was inspecting a pressure gauge at the aircraft door and inadvertently deployed the escape slide. Passengers deplaned aircraft.

Narrative

We had completed boarding the customers on the aircraft. I was Flight Attendant A working in the G5 galley between aircraft doors 4L and 4R. Since we had concluded the boarding process; I was securing the carts and all galley latches in preparation for departure. When unexpectedly; the Captain made a PA announcement informing everyone (customers and flight attendants) that unfortunately the aircraft was found to have a discrepancy and was no longer safe to fly. Everyone was told to gather their belongings; deplane the aircraft; and proceed back up to the gate lounge area in the airport terminal. A few moments later; I received a call on the interphone from FA B (who was present at door 3L) who informed me that an aircraft Mechanic had come to inspect the pressure gauge at the 3L door. In the process of performing that inspection; the Mechanic opened the aircraft door at 3L; which in turn caused the slide to deploy outside. When I subsequently walked up the cabin aisle to deplane myself; I observed that door 3L was completely open with a yellow and black barrier strap attached horizontally across the middle of the door. That was the extent of my involvement in this slide deployment incident.Suggestions: Verify if the existing procedures for performing inspections of the air pressure gauges at aircraft doors (and other similar or related aircraft door inspections) are sufficiently adequate and indicated to be done in a manner that minimizes risk to all involved (mechanics themselves; crew members; passengers and ground personnel working on the ramp).