The word you're looking for is probably *eavesdrop*:

> Secretly listen to a conversation.  
*‘my father eavesdropped on my phone calls’*

<sub>(source: [Lexico powered by Oxford Dictionaries](https://www.lexico.com/definition/eavesdrop))</sub>

@Zikato reminded me of [this scene](https://www.youtube.com/watch?v=MpOF1NHowdQ) from The Fellowship of the Ring where Sam is caught eavesdropping (around 2:45) on a secret conversation between Gandalf and Frodo.