Monthly splash from left rear wheel well (?!

Hi All!

Even my wise Prius whisperer is clueless about this one...

I have a 2005 Prius. The following problem occurs only in my left rear wheel well. So far as I can tell, it has no correlation to recent rain, or to where I've been driving.

Roughly once per month, a whitish substance splashes up in front of the gas tank door. It's very, very difficult to get off. I can see the splash inside the front and back of the wheel well, but not at the top. I can also see specks of white on the tire.

I park in a well-maintained condo parking lot and always in the same space. No-one parks to my right, and I've already discounted anything coming from the car to my left.

Any ideas?

 

No, he's quite a distance away, so we chatted by text. He says he doesn't know of anything in that area that would cause the issue. He thinks it's road splash. But it has happened 3 times, each time splashing the same area, and each time with the same whitish liquid that can't be wiped off.

 

Haha! THAT I could wash off!