Somebody’s ears are flapping
Definition 1 of 1
Pro Tip 1/3Privacy Concerns
Used when privacy might be compromised because someone could overhear a conversation. 
Lower your voice; somebody’s ears are flapping, and this is confidential.
Pro Tip 2/3Informal
It's casual language, so avoid using it in formal situations like business meetings or academic papers. 
They lowered their voices, joking that somebody’s ears are flapping nearby.
Pro Tip 3/3Figurative
It's a metaphor; no one's ears are actually moving; it's just implying that they're listening in. 
Careful what you say about the boss; somebody’s ears are flapping around here.