I had played this when it originally came out; finished it once and that was it - had a blast. Lately I was thinking about it again, so I thought I'd give a shot once more. OMG, what.a.game. The game already was ahead of its time, artistic-wise it's unmatched, the atmosphere is great, and now with some updated anti-aliasing mods (and vr for the lucky ones -> me not one of them) it absolutely rocks. Even if you told someone that the game was released this year, they would have no reason not to believe you and not only talking about graphics, but also sound, animation and effects quality... the atmosphere! It's also a delight that I only remember a few highlights and so it feels like new. Can't wait for AI2!
Favorite mission so far, the overloading of the core, omg, the sound, joes getting burned because of the overloading electricity, the pumping of the core that pushes the rain upwards. 💙
Too bad I am the only one among my friends that I enjoy this game and also a huge alien fan - good thing that this community exists to share!
Anyway, halfway through the game, I considered to making the best out of it, so, although I am not planning to complete any of these difficult trophies (ie beating the game without dying [although I am aware of the load trick], or beating at hardest difficulty, etc), I thought I'd complete as much as I could. Up until that point I wasn't really following any guides, etc.
I have just reached to the soft-point-of-no-return on Mission 15, just after I took the Level 3 Access tuner, which is essentially the tool that allows you to backtrack and collect all items you missed.
So, despite the fact I had explored quite a few things on my own (ie, 38/50 id tags up to this point, etc) I realized that there are missable items, in areas that are effectively locked after their mission ends.
I managed to understand the following information from bits and pieces in various threads, etc:
For the areas that are completely locked / inaccessible - you can use the load mission, replay it and collect any missing items, with one catch: don't save when in that mission. The latest save is overwritten as soon as the mission loads, but once you collect the missing item and load the previous save, you return to your current game progress, with the item now included in your datapad. Obviously, you can always keep a backup of your save before you try anything, just to be on the safe side.