For my mother in law to enjoy hearing us have sex?

As of right now my wife and me share a apartment with her mother(it is my wife's apartment,not her mothers). Regardless of the fact during the evenings or the mornings when my wife and me are having sex we notice that her mother starts to not only come close to our bedroom but also hang around there which puts us in a uncomfortable position. Its obvious she likes to hear us have sex because she is smiling more than normal the next day! Is this normal?