Whether the Dashwood family has guessed he is engaged, might be his immediate reaction, but he can not have wondered that for long. The family's behavior to him could not be attributable to any such discovery.

However, I think this secret does allow for his outburst in ch. 5 to be so far over his usual calm manner. Elinor will be sharing the county with his betrothed when he next visits Lucy Steele! Poor Mrs. Dashwood is so sure his agitated manner is only based on his affection for her eldest daughter! Edward must have many associations with Devonshire, before this tale ends...