I know I havn`t seen one yet, but could it be one of the four option buttons on the fascia. The brochure and the website list prewash under options and there just happens to be four of them listed in the brochure. Or I could be barking up the wrong tree of course 😉
I didn’t want to be negative but I have to agree about the manual being ‘rubbish’. Luckily the dishwasher it replaced didn’t have a pre-wash and I didn’t sell it as having one. Shall have to wait and see if the customers reads up on it. hopefully not. 😉