When my friend was about 11 or 12, she went to an all-inclusive resort with her family. This was one of those big places, with several pools, restaurants, kids areas etc. Her dad buys walkie talkies – one for her and her brother, and one for the adults (this was before cellphones were a big thing), so that they can stay in touch if the kids want to do their own thing.
One night, her parents and her brother go down to the restaurant for supper. My friend isn't feeling well, so she asks to stay in the hotel room and watch a movie. They agree, giving her the walkie talkie for emergencies. She's up there for an hour or so, checking in periodically with her dad (just to say hi, ask what they're eating etc).
She starts to feel really sick, and says to her dad "hey, when will you be back to the room?". Her dad answers back "We're coming up right now – sweetie, I forget the room number, could you tell me what it is?".
As soon as she's about to answer, her dads voice cuts in "Hey Chirp, we're coming up right now, we brought you some cake, see you in five".
She knows the second one is her dad, because he always calls her Chirp. She locks the door. When her parents get back, they say they haven't talked to her all night.