Unlock Ending Sequences
When you perform the following tasks the corresponding alternate ending will become unlocked.
Ending 1 - After disarming Demitov, turn around and use the TMD on the portal. Neutralize past Renko (headshot).
Ending 2 - After disarming Demitov, turn around and neutralize Barisov.
Ending 3 - After disarming Demitov, neutralize both targets.
Upgrade the TMD to it's maximum level and then quickly break the vent behind the upgrade machine. If you now follow the crawlspace you will eventually come to the underground wheel from Lost.