If you don't see the heart then you are not logged in. You need to be logged in for the system to know who you are so that when you click the heart, it knows who's favourites page to add the listing to. The hearts are grey and turn orange when you click them to save them to your favourites. If you are looking through the home owners listings pages and you come across a listing with an orange heart, you know that you have already added it to your favourites.