I was contacted today because someone in the accounting office heard noises in the wall. She assumed it was a squirrel and was concerned that I should evict it humanely. I told her that killing it shouldn't be necessary, asked some questions about where she heard it moving, and set about to inspecting the building. No holes chewed on the outside, no damage to the heating system visible. I set up a ladder, and climbed up on the roof. It was very cold, but not as nasty as yesterday, and fortunately there was no snow or ice on the roof.
This building is actually three pre-made buildings--trailers, really, attached together at the far ends. Between the three units are two big gaps, completely inaccessible except from the roof. I looked down into the one adjacent to the accounting department, and started to think about how to move the ladder from where I put it to get up down into the void between the units. Suddenly there was a flurry of movement on the leaf litter on the floor of the void. I admit it startled me, not a feeling I like when I'm standing on a roof.

Edited to add: Helpful illustration.
Instead of a squirrel, there was a duck. A female mallard, spooked by my sudden appearance above her, was in a panic. I stepped back for a minute and thought about what to do. I made a couple radio calls, and soon I was waiting for a zookeeper (Colleen) to bring me a carrier (the kind of pet carrier in which you might bring a cat to the vet). Unfortunately, the space was so narrow that the carrier wouldn't fit in it. Colleen went to go get a net (a long fishing net, which comes in handy when catching ducks and other small animals), and I went in and got a cardboard box from the accountant who had contacted me.
Picking the ladder up with one foot, with my body straddling the edges of the eaves of the two buildings, was the next exciting task. Once I figured out where to set my weight, it wasn't hard--it was a very light aluminum ladder. Catching the duck wasn't too bad (I didn't end up using the net, there wasn't really any room to use it). While down there I noticed frozen green bird scat, suggesting to me that the duck had been down there for some time with little to nothing to eat. Cradling her under one arm, I looked around quickly for squirrel holes, finding not much but a loose piece of siding where a piece of molding had fallen away. It could conceivably have been a way for a squirrel to get into the wall. I told the head of facilities, and he sent the carpenter to fix it.
Colleen helped me get the duck into the box, and then down the ladder again, and I took the duck back to the zoo hospital. The duck was checked out by the vets and vet techs. They found it to be a little thin, riddled with lice and mites (I hurriedly put my gloves in the dryer when I heard that), but otherwise healthy. They also found a puncture wound, several days old, that was consistent with a hawk attack. The vet suggested that maybe the duck was grabbed by a hawk, got away, and dropped down into the space between the buildings, where it spend the better part of a week. There wasn't enough space for the duck to fly out without striking the walls. They gave it some fluids, and kept it in a quiet empty ward for a few hours. When they brought it across the street to the park, they let it go and the duck exploded into the air, flying away with new found strength.