Technician reported aircraft settled on to a ladder that was being used during Crew O2 bottle servicing. The ladder damaged the lower fuselage that was obscured by a belt loader that was moved into place to load the forward cargo compartment and went undetected.

Narrative

Aircraft damage to Aircraft X on Gate XX; Log Page XXXXXXX - Date I was working on Log Page XXXXXXX to service crew oxygen and used a step ladder to get into the forward pit to access the oxygen bottle. There was a belt loader positioned in the forward pit and since there was no ramp personnel around I moved it back to get access to the pit. When I approached the pit with the ladder; I did not notice any damage but I was also focused on the front of the pit as that is where the oxygen bottle is located. I opened the forward bulkhead pit panel and removed the oxygen bottle and handed it out to the two Mechanics who were assisting. After the oxygen bottle was serviced they handed it back to me and I reinstalled it. Once the pit panel was closed back up I exited the pit and pulled the ladder away from the aircraft; I did not notice any damage but I also pulled the ladder forward away from the damaged corner. I did not hear any popping or cracking while I serviced the oxygen and the ladder was not stuck under the aircraft or had any resistance to movement when the ladder was moved. The aircraft was being loaded with passengers and baggage in the aft pit while we were servicing the oxygen. The ramp crew then needed to pull the belt loader back up to the aircraft after we were finished with the oxygen service. The damage would have been directly in front of the driver of the belt loader but they did not notice the damage when they approached. The damage was not identified until after the baggage loading was complete and the belt loader was moved away from the aircraft and the airplane was ready for departure. I reported the possibility of the ladder causing the damage to Person A as soon I realized that it was a possible that the damage could have been caused by the ladder as the aircraft moved as a result of the loading of the aircraft. I am not sure if the ladder or the belt loader caused the damage; so it is hard to Identify what could have been accomplished differently to prevent the damage. If it was the ladder that caused the damage then being more aware of the surroundings and what is happening in concurrence with the oxygen servicing. If it was the belt loader then the ramp crew would need to provide a how and why.