I have a 2006 Prius and have been fighting this smell for a couple years.
Here is what is causing the nasty smell.
Under your back seat are the batteries.
Try smelling in your trunk by the spare tire.
These are enclosed in a steel box and vented to the rear passenger side of the car's wheel well. On top of thes batteries are (2) plastic tube vents that connect each of the battery cells together. Part # 28876-21020 you need (2) at $80.95 ea. You can not see these vents without taking the rear seats out and taking the steel battery housing apart. Real pain.
Even though the batteries are under warranty Toyota says these are not covered?
These tubes get sticky and smelly as they degrade. I took them out and 5 days latter the smell is greatly reduced. It takes time because there is some sticky material left on the batteries and this need to evaporate.
Toyota took 3 days to get the 2 parts in and when I went to install them I found one of the new vents was sticky and smelly. I am waiting for a replacement. They must have a production problem on these parts.
We need to fight Toyota on this repair. This should be covered under warranty. Who knows what chemical we have been breathing. They are playing dumb. There are postings about this on internet so this is not an unusual thing.
I think you are better off having the dealer do this work rather than doing it your self.