Strange problem on production server 'tmp' deleted

Hi all,
the last days I have a strange problem on my production server (Ubuntu).
My project files are located under a subdirectory /app
Under /app there are all the project files including the _lib folder where usually the tmp folder is located.
Since some days the tmp folder disappears from time to time. What to do now?

To localize the problem I need to know when exactly the tmp folder is deleted and what causes this. Does anybody know if there are log files where the information is stored?

Joe