When it comes to finding synonyms for "behind closed doors," there are a number of options available. One possibility is "in private," which suggests that something is happening outside of public view or scrutiny. Another option is "in secret," which implies a greater level of secrecy or confidentiality. "Privately," "confidentially," and "off the record" are also good choices, conveying similar ideas of privacy and secrecy. Other synonyms might include "unobserved," "out of sight," or "in the shadows." Ultimately, the choice of synonym will depend on the specific context in which the phrase is being used, as well as the tone and emphasis of the overall message.