[TYPO3-content-rendering] apache error messages flooded with "no_cache ..." entries

[Florian Schwaiger]

The message ist "no_cache has been set before the page was generated - safety check. Caching is disabled!"

Curiously, I cannot find any TypoScript snippet or code call to set_no_cache in my project.
I have also tried to access every subpage on our website, but am absolutely unable to trigger the error myself, even by calling uncached dynamic pages.
It happens once every some minutes but from totally different users and without a fixed interval.

What could possibly generate these errors?

Regards
Florian