Our Siemens dishwasher retains water and smells

I would check the bottom of the dishwasher to see if there are any bones, old food built up as some of these collection systems have a small disposer built into them.
If you find any food, it's probably that.
Also, you can make sure that the drain line is the proper length and high enough as this is a common problem. Do a good search on proper drain height for dishwasher.
You may run an empty load of dishes with a cup of bleach added to eliminate the odor as well. Bleach will kill any bacteria causing the smell. More than likely the drain hose wasn't installed correctly.