In 1990, the Census Bureau conducted two operations designed to include homeless persons in the census: an enumeration of the occupants of emergency shelters, and a late night enumeration of street sites identified by cities and census offices as places where homeless people congregate. To assess the street enumeration, the Census Bureau sponsored independent studies in which unobtrusive observers were stationed in a sample of street sites. The observers reported their observations and experience of the enumeration process in debriefing questionnaires filled out immediately after the conclusion of the street enumeration. Data reported by the observers are applied here to fit a plant-capture model, which is an alternative to the classic capture-recapture method of estimating the size of a population. This method assumes that the plants (in this application, the street observers) have the same capture probability as other members of the target population. The plant-capture method has potential application as a method to evaluate coverage of the homeless population and other populations for which the assumption of closure is questionable. The paper analyzes the data to develop various estimates of the capture probabilities, and assesses the strengths and weaknesses of the method as a potential source of coverage estimates in future enumerations of the homeless population. The paper also discusses weaknesses and uncertainties in the street observer data, and evaluates how the quality of the data may affect future attempts to base coverage estimates on similar data from observers or plants.