Lake Mungo/Fridge

Fridge Brilliance

In-universe example, during the credits, most of the photos or footages seen during the film ( even the fake ones) that appeared completely innocent, such as Matthew's birthday party, all contained the extremely well-hidden ghost of Alice.

At the end, the family think that by solving the mystery like Alice wanted them to, they've put her spirit to rest, as she doesn't seem to be haunting them anymore. But as the intercutting interviews with her and her mum show, and the photo of her staring wistfully out at them standing on the lawn, her problems weren't solved at all. It's just that because they've gotten closure and moved on, they can't feel her there anymore.