But this is not a Maemo, but SailFish OS on a Xperia 10-II
It looks like it should be someplace under /sys/devices/platform/soc/soc:tlmm_gpio_100_reg/subsystem/ but that is quite the rabbit hole and I cannot find it.
Go into settings, on the ‘system’ tab and scroll down to ‘About Product’. Go into this page and look for ‘Build’. Tap on this 5 times and the CSD tool will load.
Go into ‘Individual Tests’ and select ‘Proximity Sensor’.