Update the following day...
Interestingly, the soap dispenser was triggered this morning when my mum opened the blind just above the dispenser, which was closed last night when the problem started. Opening the blind let in a lot more natural light. So it can be triggered by increase in light...
What about clear contact used for covering library books etc? This stuff can be very durable, and you should be able to find a low-sheen variety. You'd have to make sure it's applied perfectly the first time, which is not hard with some practice.
Otherwise, I would just sandwich the photo in...
I have a Dettol automatic soap dispenser (see fig. 1) which uses a little propellor-type pump to let out a small amount of soap when you put your hand in front of the passive IR sensor.
It's been working fine until just before, when I turned the lights off in the kitchen (night time, so...