IN THE LATE 1990s, a flood of biotech/health science companies chose to locate to this city, many lured by cheap rent. The growing number resulted in a hub that now includes more than 20 companies -- mostly located in the industrial area off the Highway 92. But in the past year, it appears the area has not gained any new companies in this growing industry. Now, two major players -- Cholestech Corp. and Kosan Biosciences -- are looking at the possibility of leaving the city. All this comes at a time when the City of Hayward appears to be doing nothing to attract or retain biotech/health sciences companies.