Mysterious Germany: the random Oil Spill sign

One of the most puzzling and enduring mysteries the blogster has lived with in Germany comes in the form of a mobile road sign.

In our hillbilly* county, it comes as a triangle, white background, red band along the sides, with an exclamation mark (!) in the central white part plus the word "Ölspur" (oil spill) underneath on a separate, smaller white sign.
The combination sits on a metal tripod and warns drivers of oil on the road.

The triangle, shown here in all its glory, is the generic "Danger" sign. American friends love it.

There is even a photo of a dear friend in a slightly, let's call it naughty pose with such a sign. Straddling it, would be a more technical description.

Despite the fun you can have with the exclamation mark sign itself, our experience is limited to the combination indicating an oil spill.

We never saw an oil spill when the sign was out.

The first few times we encountered the warning while driving on the roads of our hill country, we dutifully slowed down, looked for the spill with the requisite orange colored mix of whatever is used to soak up mineral oil, to contain it at the source.

The conversation typically went like this:
"There's an oil spill."
"Okay."

One or two hundred meters later:
"Did you see anything?"
"Nope."

After several occurrences of this, we began to wonder what was going on.

This being Germany, we redoubled our effort to detect spills. In some countries, the road crews will bring out signs when a spill is large, oil drum size or so. We figured Germans might be more cautious.

Still nothing, not even a table spoon of escaped oil anywhere, no multicolored little indication on the road surface.

Predictably, seeing the oil spill sign became a running gag.

"Oh, look, oil spill, hehe."

Years into the mysterious appearances of the oil spill sign, we resigned ourselves to the only plausible explanation.

We decided it was the county road maintenance crew's idea of fun and job enrichment. As in, well, nothing is happening, so why don't we go and put up the oil spill sign somewhere.

Deepening the mystery was a fact we noticed after a while: the sign always came out on nice, sunny days.

"Guess they keep it inside in winter, so it won't get damaged by road salt and ice."

Then yesterday happened.

"Oh, look, the oil spill sign."

"Wait, that's an oil spill. See the orange stuff, it's a real oil spill!"

The exclamation mark in the utterance neatly matched the one on the sign.

Of course, come summer, we'll be scrutinizing every display of the oil spill sign even more closely.

* The blogster self-identifies as a hillbilly, cause Ozarks, you know.