Americans at risk in Germany?

From the U.S. embassy in Berlin:

U.S. diplomatic and consular facilities in Germany are increasing their security posture. We are taking these steps in response to a heightened threat situation.
The U.S. Embassy encourages Americans in Germany to increase their vigilance and take appropriate steps to bolster their own personal security.

I wonder where this could be coming from. I wasn't aware of this, incidentally, when I posted the photos below, so I'm not suggesting there is any kind of link here.