“In the prologue’s icy river, under the bridge, a frozen visitor lies. Miss it once, fail the mission you will. Miss it twice, shame on you, it is.”
Details: During the Ludendorff prologue, veer off-road before the bridge and find an alien encased in ice. Revisit during Bury the Hatchet for a second look.
PC Pro Tip: Use OpenIV to extract the alien model for custom mods (like turning it into a Pegasus vehicle).