1. To allow something to remain within something else. A noun or pronoun can be used between "leave" and "in." Make sure you leave in these curlers for at least an hour or your hair will go flat again in no time. He left the key in the ignition while he went into the drugstore.

2. To go without taking someone or something from within some position, place, or thing. A noun or pronoun can be used between "leave" and "in." I left the kids in the play room while I went up to have a shower. I think I left my phone in the hotel.