Fault found, on a wee infrared sensor board. Had hoped that just cleaning it up would be enough, but the combined water and soap leakage on to this board had eaten through several areas of copper (under the green lacquer), and removed the tin plating from the pair of diodes. Judging by the lack of any voltage present on the board after cleaning it all, it's likely that a short circuit here has damaged something else on the main board.

Could try making a completely new board, or a bodge repair on this one. Need to buy another pair of diodes to try that. One of the diodes emits infrared light, and the other is a photodiode that is sensitive to infrared light. When an object, like a hand or finger, is placed in front of the pair, some of the emitted light is reflected back to the photodiode, and that is detected by the circuit, and a wee pump is operated to give out some soap... or should be. Did disconnect the pump to test it, and it still works.

Next mission, to locate a suitably teeny matched pair of these, or similar, diodes to try out.