S.,
After trying the other ideas, hopefully you will find the source of the smell, you can set out a deodorizer.
Baking soda works fairly well, but activated charcoal is even better. Because of the unusual shape there are lots of little nooks and crannies that odors can stick to on the charcoal. You can purchase a small container at a pet store in the fish aisle. Just put the little black crystals into a large pan and set it somewhere in the room.
The new show on HGTV, Food Investigators (or something like that) found that charcoal worked better than baking soda.
Even better, after the charcoal has done it's job, set the pan outside in direct sunlight for a full day and it is ready for another go at the smell. I don't know why the sunlight works, but it does.
We are using this in our house right now because our dogs had an encounter with a skunk over the weekend and brought the smell inside. ICK!
Good luck!