I have an odd setup - two dishwashers were in our home when we purchased it a year ago. Since we have two, we don't run them as often as one might and I noticed that the second one has an accumulation of water and "funk" on the bottom. From what I can see, the water is pumped all the way out but then comes back into the bottom of the dishwasher after the draining is complete.
I'm guessing that this is due to the plumbing - as you can see in the photos, one dishwasher drains just under the sink and the other (the one with the problem) drains to a second pipe with a second trap (in the back, on the left). I tried propping up the trap to create a better flow but that didn't solve it. The drain line does come over from the disposal fairly high up behind the sink.
Adding to the complexity, we'd like to install a disposal if we can fit one - not sure if that impacts things much but thought I'd mention it.