As with most shooter games, there is not exactly any sort of side quests that are offered. We could consider the ‘side quest’ of hunting items that you do not need to progress the plot instead collecting all of them to boost your Gamerscore from an achievement. As we played this on the PC, aside from the usual ‘steam’ achievements, which aren’t bragged as much as for console players there isn’t much incentive for us to find them all and get a perfect achievement score for the game.
Items to collect may involve diary entries, concept art, enigma codes, and even ‘upgrades’. These are scattered all across the game, requiring you play it through on both timelines in order to access all the areas to find them. The more useful of the items are the upgrades, which either increase the maximum amount of health you have, or increase how much armour you pick up from an armour item on the ground, essentially letting you fill your armour back up faster. Enigma codes unlock certain gameplay modes, which again we believe are so that you can play the game certain ways in order to unlock unique achievements. A lot of the collectibles were hidden uniquely in certain ways that did promote exploring the mostly linear environments in order to find them, though aside from what we mentioned there is no real ‘reward’ to finding more concept art or diary entries unless you want to know more about the concept art or backstory to certain characters whose diary entries just happen to be scattered all over the place.