Final answer:
The strange expression on the hostess's face could signify various scenarios; context from the narrative is necessary to discern the specific cause.
Step-by-step explanation:
The strange expression that came over the face of the hostess could have been due to a variety of reasons. The question presents three scenarios involving a cobra, unhappy guests, and the realization of a cobra under the table. Literary contexts often require an analysis of situational details, character behavior, and symbolic elements like snakes or cobras which often symbolize danger or deceit. In literature, expressions and reactions of characters are crafted to represent underlying themes or plot developments. The specific reason for the hostess's expression could be better determined by considering the entire narrative context within which this event occurs.