Standing water, along with debris, such as eggshells and coffee grounds in the dishwasher could be from one of two reasons. The debris got there from not thoroughly rinsing dishes. Thus, depositing trash in your dishwasher. Discarded particles may have arrived in the bottom of your dishwasher through the drainage piping for the sink. Many times, the dishwasher discharge hose is attached to a tailpiece below the sink drain and above the trap.
When water is carrying debris or, let's say it is just plain water, 9 times out of 10 the tubing on the side of the tailpiece is where water and debris find a path to the dishwasher. That is your dishwasher's discharge line.
The discharge line is there to carry the grey water from a dishwasher to the sanitation piping and on to the sewer.
If you have a garbage disposer, the dishwasher discharge hose is attached high on the garbage disposer.
One thing you can check is to be sure the knock out plug on the disposer is removed. (This would be a step in diagnostics for a new or recent disposer installation) If not, the knock out plug will not allow discharge from the dishwasher to enter into the sinks drainage system. Also, see if the discharge line is secured above the drain under the sink. If it is not, then, any liquid carrying debris from the sink can be diverted to the discharge line and gravity does the rest to lead waste water to the dishwasher.
People who discover this condition in their dishwasher usually say they do not use the dishwasher very often. This is why they will occasionally notice standing water at the basin of the dishwasher. The water has had an opportunity to exceed the level of the internal pumping chamber where it makes it's appearance. Some people will say it is stagnate and has a foul odor and dark color. Again, water that has had time to accumulate and is carrying organic waste will do that. Need help, give Anchor Sewer and Drain Cleaning a call in Rhode Island and S.E. Massachusetts