The Oobi sensors are mechanical sensors placed around Kakoo's Box to sense when her boyfriend Oobi is passing by. When Oobi is nearby, they will scream "OOBI SENSOR OOBI SENSOR, WE HAVE FOUND OOBI. KAKOO, WAKE UP!" That signals Kakoo to jump on Oobi.
The Oobi sensors are not seen, only heared. They sound like a deep, bellowing male voice.