When it comes to describing someone's hidden intentions, there are a plethora of synonyms that one can use instead of the phrase "hidden agenda." The term "ulterior motive" implies that someone has a secret plan behind their actions, while the phrase "secondary purpose" indicates that there is an underlying reason for something that is not immediately apparent. "Subtext," "underlying message," and "subterfuge" are other options to describe an unspoken motive or intention. Additionally, more literal synonyms like "concealed plan" or "covert objective" can be used to convey a sense of secrecy in someone's motives or intentions. Regardless of the word choice, the concept of a hidden agenda remains one that we must always be aware of.