DolphinSearchBox::init() leads to a call to DolphinSearchBox::isIndexingEnabled() which can generate the following terminal warning depending on the state of the m_everywhere checkbox and whether or not m_searchPath has been initialised:
QFileInfo::absolutePath: Constructed with empty filename
From what I can tell, DolphinSearchPath::searchPath() returns $HOME or m_searchpath depending on the last used search settings, so the warning can seem to appear randomly if you don't use the search feature often.
It can be prevented by a simple check whether the current search path is valid or not. searchInfo.shouldBeIndexed() returns false for empty paths so this change is transparent. As far as I can tell, m_searchPath == QUrl() is not an anomaly but not special-casing this value is.