Hatdrm and K2 - hi, I haven't had the "fortune" of dealing with dead mice in office walls - my last critter was a small opossum. My suggestion for finding the critter would be to use a stepladder and strong flashlight - use the ladder to get above the dropped ceiling if there's one, and use the light to look down between the drywall sheets. Many times, the walls don't run to the true ceiling and since the drywall's mounted to metal studwork, it's not capped at the top so you can look down through to the true floor.
Otherwise, you'd end up having to locate and use some type of borescope or remote inspection device that puts a mirror or camera on the end of a flexible wand and has a light source to illuminate the area you're trying to view. (You might even find something like this at a local tool rental place...) If all you have is an inspection mirror, you can shine a flaslight on the mirror and the reflected light would light up some of the area. This would work if you're able to gain access into the wall area by removing a telephone outlet plate where no box is behind the plate (in an office environment, you'd not normally find this unless someone ran the line in a big hurry after construction was completed).
When it comes time to remove the critter, recruit a handyman or DIYer to help out. If the wall has wallpaper, they should be able to cut the paper to peel it back, then cut through the drywall in such a way as to only need some spackling to put it back in place afterwards. A little wallpaper paste will put the wallpaper back down, and then it's just letting the smell dissipate. When I removed the opossum, our civil engineers (I'm in the USAF) gave me a professional-strength smell neutralizer that was definitely orange-based (like the cleaning stuff you see in the infomercials). I used some of that on a rag to wipe down the area where the carcass was laying, then sprinkled some on a paper towel and left that inside the wall. The orange smell was the only thing you could detect and it dissipated after a couple weeks. The container didn't have a brand name on it - sorry!
I hope this helps you some - good luck! Jim D/Heathsville, VA