The word you're looking for is probably eavesdrop:

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

_{(source: Oxford Dictionaries)}

@Zikato reminded me of this scene from The Fellowship of the Ring where Sam is caught eavesdropping (around 2:45) on a secret conversation between Gandalf and Frodo.

The word you're looking for is probably eavesdrop:

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

_{(source: Lexico powered by Oxford Dictionaries)}

@Zikato reminded me of this scene from The Fellowship of the Ring where Sam is caught eavesdropping (around 2:45) on a secret conversation between Gandalf and Frodo.

The word you're looking for is probably eavesdrop:

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

_{(source: Oxford Dictionaries)}

The word you're looking for is probably eavesdrop:

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

_{(source: Oxford Dictionaries)}

@Zikato reminded me of this scene from The Fellowship of the Ring where Sam is caught eavesdropping (around 2:45) on a secret conversation between Gandalf and Frodo.