B737-800 ground personnel reported discovering the flight load was already planned and closed by an unknown person; who also noted HAZMAT had been loaded on the flight.

Narrative

I was scheduled to load plan outbound Aircraft X to ZZZ at Gate XX. After completing an inbound at [Gate] XY I went over to [Gate] XX to begin the flight load. I open the card and saw that the load had already been planned. No one else on my crew was card qualified. So; there was no one to refer to regarding the flight planning and I was not given a physical DSTG (Departure Staging Guide) card by whoever planned the flight. After loading the cargo onto the aircraft; I could not close the flight because the DSTG was still open. After opening the loading form; I discovered that the flight had already been closed by someone else. This person was not a member of my crew nor were they present during the loading of the flight. There was no verification between this person and I at any point or time. My number still remained on the loading form as the Loader of a HAZMAT item. The person that closed this flight could not have known that I loaded a HAZMAT item on the aircraft because that person was not physically present to have seen me do so.