As with most "secret passages" on campus, they aren't actually secret, but they are handy and not well known.
If you are trying to walk to the Building 43 cafeteria from the southwest, the usual route is to take the unnamed service road that runs between Building 43 and the highway, then go up the gravel emergency driveway that runs between buildings 43 and 44.
The secret way to get to the cafeteria is to enter the Building 43 parking garage via the small loading dock that sits opposite the pedestrian crossing.
Go up the stairs, and then turn left.
Look both ways before crossing the main driveway, and then continue through a tall, narrow passageway to another driveway.
Follow the second driveway, past the loud ventilation fans, and then you will see ahead and slightly to the right of you a wall painted purple with the message "Stairs to…" Follow that wall. (If you forget to turn, then you will continue through a second tall, narrow passageway to the large loading dock.)
After you turn the corner, you will see that the rest of the message reads "… Cafeteria." Follow the path down the slight ramp and use your cardkey to unlock the door at the end of the path.
When you go through that door, you will find yourself in the cafeteria, via a door that you probably didn't even know existed. (And the message on the purple wall is a lie: There are no stairs.)
This shaves a bit of distance off the outdoor route, but a major benefit is that this version is inside the parking garage, so even though it may not be indoors, at least it's covered, so you don't have to deal with the rain, and it's not quite as cold.