Yes- that is correct. It used up all the clean water but there was no dirty water in there to pour away.
We have just tried it now. We have filled the tank in tank with warm water and made sure it was sitting in properly. I switched it on presses the trigger to put some shampoo on the carpet and released it. Again no water was to be seen at the front of the cleaner as usual but the carpet is wet. No dirty water is in the tank. The Tank on Tank is a brand new replacement as the old one's water holder had perished. Could the new tank be faulty?