I gave the medicine to the retired agent and after that I kept hearing a voice saying something about some documents and that his behind me. Now without spoiling, is he supposed to be right behind me or do I have to search for him? I checked the area and I haven't found him yet and when I left the building the voice stopped talking. I still see the message "please take the documents now".

Nevermind, I didn't realize it was a guy in the control room.

Hahaha, I had the same issue as you when I heard that voice.
It took me a lot of time to realize who was talking to me.
Maybe it can prompt us that we have to get off the console and turn around.

In game, someone had told us that any new letter we found there would transmitted
a copy back to our facility, but I didn't expect that the initial letter which was not found
by us, was transmitted to the facility and an agent was handing it over behind us.