Koko is a native Japanese word meaning 'here' that is almost always written in hiragana. In the past it was written in kanji as or , both actually Classical Chinese words meaning 'this place', but these are seldom found nowadays.
There is a rather peculiar usage in sketch maps to write koko in katakana as . This is a little akin to writing 'x marks the spot' on a map in English.
This usage can also be found in signs, as in the sign below indicating the location of the advertised premises.
A Google search of the Internet in August 2003 revealed the following distribution of usage:
Results for were too heavily influenced by unrelated words that contained the two letters . To show the general prevalence of koko written in katakana, searches were carried out for the specific forms koko ni, koko ga and koko e.