I got this on my 5th attempt. Most of the guides above are good to a point but didn't unlock the achievement.
Explorer, as the names suggests, is granted for exploring. Some of these explorations result in ammunition, some result in Discovery Points (screen flash and noise), some in nothing at all.
It is NOT linked directly to pick-ups. The achievement popped for me on Outpost having jumped out of one of the windows to use a wooden boardwalk (this wasn't where the game wanted me to go). It popped without picking anything up or doing anything else bar enter this area.
So, take what you want from the other guides, but I suggest following this excellent guide to a tee - do absolutely everything mentioned and you will get the achievememthttp://www.gamefaqs.com/xbox360/935091-metro-2033/faqs/59427
- full credit to Matthew Y Brown & gamefaqs
I would also suggest a little extra exploring as I found a couple of areas not included in the guide. It is also worth mentioning I did not knock every door, or listen to every conversation. On level "War", "Frontline", I sneaked through the level first picking up 3 Discovery Points, then killed absolutely everyone and exploring every inch top-side. I then used the pipes to exit the level via the manhole.
Given this was my 5th attempt I had also deleted every single save file for the game and started as new on easy - there is no way to verify whether or not this affected the achievement popping.
Archived version of the guide - http://web.archive.org/web/20111220083821/http://www.gamefaq...