Just posting my recent problem and describing the solution in case this occurs to others.

I bought the XC90 used a few years ago with a bit over 90k miles, now has a bit over 100k miles on it. I had no idea what the history on maintenance was, so had it thoroughly checked over last summer, did timing belt, some brake work, some other driveline work, etc. All done by a good independent mechanic.

Three days ago it was raining, I did some errands.

Two days ago, noticed brake lights were on while the vehicle was parked and locked. It started up, but the battery was low. Drove it a bit and I could hear water rushing somewhere in the vehicle when I stopped and started from a stop. Drivers side floor was wet, but I did not know if that was leakage or from my feet getting in and out when I drove it in the rain. Put it on a battery charger. Passenger side floor, back seat floor, the compartment in back where the battery is were all dry. Did some internet searches, brake light switch and a brake light relay were mentioned on several posts.

Yesterday I called the shop that I have used before when they first opened, made an appointment to bring it in this morning first thing. Later yesterday, drove some errands. Started getting more warnings, brake anti-lock failure, the trip meter stopped showing distance, a few other weird electrical issues. Left it on the charger overnight last night.

This morning, battery charger did not show any amps on the gauge, I expected it to show 5 amps. Then looked behind the vehicle, brake lights were off. Drove it to the shop, all electrics were working fine, but audibly, there still was water somewhere.

Mechanic found the drain hose from the sunroof on drivers side had come disconnected. And he managed to remove a couple gallons of water from the vehicle. Also fixed the hose. He concluded that water from the drain hose had leaked into electrics under the dash and caused the electrical problems. Weather here has been cool (30s to 50s F), so not having any mildew problems, at least not yet. Left windows and sunroof part open today, plan to do that for a few more days.

So, now a few hundred USD poorer, but no more water rushing sound. Electrics are working ok.

Before I bought this I did some internet research and learned that the sunroof drain hoses can come disconnected as a common failure, but I had not even thought to check that because the brake lights were on and that was distracting me from thinking about drain hoses. Instead my thinking was all focused on electrics because the brake lights being on was the first problem I noticed. And nothing in my internet search lead to checking for water in the electrics.

 

Keep in mind - there is up to 4 inches of foam rubber bonded to the back of the carpet - the ONLY way to get the water out of that sponge - is to remove the carpets.

I'd bet if you pulled the threshold up (plastic piece you step over when door is open between the body and the carpet) and felt underneath the carpet - it will be wet and eventually will not smell nice.

 

Thank you for the warning.

Right now, it is 32 degrees (F) outside with dewpoint in the 20s, forecast high today is high 40s, freezing tonight again. Should remain cool for several more days. I am hopeful that it will continue to dry out with the dry air and refrigerator like conditions.

 

It has been cool enough here that it dried out without any smell by leaving two windows cracked and the sunroof partly open on days when there was minimal chance of rain.

If it had been warmer, I probably would have used an electric fan to blow air on it, I considered that. But I was hoping that the cool and dry conditions would suffice.