Portuguese equivalent: “segundas intenções”.
“She agreed to go to the mall with you? I’m sure she has a hidden agenda. She must want new shoes, or something”.
“The Congressman had his own hidden agenda when casting his vote: he was trying to benefit his partisans with it”.
Ainda está difícil entender a expressão? Que tal uma aula?
PS: To my reader, Roque: I swear this post was ready before you mentioned the expression on the comments!!