I've wore 'just in case diapers' as you call them basically ever since I finished potty training. This didn't start as a fetish or even a hidden desire for me, it became that as I got a little older because the attention you get wearing nappies/diapers as an older kid, good or bad, can be very fulfilling.
I was finnaly properly potty trained at 5 but I still wore nappies to bed and when travelling anywhere or when I was sick. Then it turned to just when travelling anywhere or sick. Then to just when I was sick, or travelling on long jorneys. Then at 9 I started bed-wetting so it went back 2 to bedtime and whenever travelling anywhere or sick. This led to me starting to feel much more comfortable when in a nappy than in underwear and I started to leave them on in the day if they werent wet after waking up, even thouh I don't wet when awake. My mom thought it was me being lazy and most of the time had no problem. Then one time my mom told me to change into a clean one for bed because it had been a hot day and she wanted to make sure I didn't get a rash, but when I took off the nappy she found poop marks in it and started having me wear nappies in the daytime because of this. Then when I turned 11, since I had stopped wetting the bed for a while, I was allowed to stop wearing nappies altogether, but I told my mom I didn't want to. I don't think she understood why or if she does to this day, but she didn't make a big fuss nor have many people I know.