Is there sin upon the husband for concealing his temporary marriage from his coworkers?

A religiously valid marriage is one that fulfills the five conditions, among them the testimony of two upright men. If these conditions are met, then the marriage is valid, even if no one knows about it other than the spouses. This does not fall under the forbidden secret marriage (Nikah as-Sirr), according to the most preponderant view among scholars. Therefore, there is no harm in keeping this marriage confidential.