XC90 Rain Light Sensor Issue

Sounds like a sensor fault alright. Have you checked for any obvious damage to the windshield around the sensor area? Any sign of delamination or bubbles under the gel pad? That's a common cause.

Bubbles under the gel pad? I haven't specifically looked for that. Where exactly should I be looking, and how obvious would it be?

Yeah, it's where the sensor 'reads' through the glass. It might not be immediately obvious, but look closely at the area where the sensor sits against the windshield from the outside. Small bubbles can distort the sensor's view. If you suspect that, I'd get it checked out by a workshop.

Thanks for the tip! I checked, and you were spot on – tiny bubbles under the gel pad. Took it to the shop; they replaced the gel pad. Cost me 62€, and everything is working perfectly again! Really appreciate the help!