Ok, for everyone who is interested, too, where the clud comes from:
I have asked on the AOO mailinglist and have become the answer.
http://mail-archives.apache.org/mod_mbox/openoffice-users/201309.mbox/raw/<522C7B3F.4010100%40t-online.de>
it is the "cloudCallout" from presetShapeDefinitions.xml from ECMA-376.
It is part of the standard specification of OOXML.
Greatings
theuserbl