Not supposed to be one-way - good catch. In the original, all three Silo doors lead back to their matching doors at HQ. The Silo's three doors are stacked almost exactly on top of each other, so our game mistook door No.3 for a copy of door No.2 and quietly removed it. Doors 1 and 2 were just far enough apart to escape this. Door No.3 is back, and now takes you to door No.3 at HQ. This is fixed for the next update but not live yet, so please re-test after the next patch.

Now live: Silo door No.3 takes you back to HQ again. Refresh the game to get the update.