Clear water returns after cycle is complete

At first I thought I was crazy or negligent. I opened the dishwasher after the cycle, but didn't empty it immediately. The next morning I had an inch plus of water (clear) in the bottom of the dishwasher. I monitored this for two weeks. If there was water left, I could choose a cycle after I emptied the dishes, and that cleared the water out. I have checked the air gap, cleaned out the food filter, cleaned the holes in the arms that spray water with toothpicks and tweezers, cleaned out the hose that goes into the disposal, used drain cleaner in the disposal. Today I did the baking soda, vinegar, and hot water drill to clear a clogged drain. Twice. Seemed to work a bit at first. After I ran the dishwasher with a dishwasher cleaner agent, the little tub leading to the outtake was finally empty. Now it's been two hours and clear water is regurgitating into the basin. So my earlier observations, where I thought first the water was not there, but then it was there the next morning, are probably correct. Do I need a plumber or a dishwasher repairman? GE model, about 4 years old.