Bee is searching for her mother, literally, sure—but also figuratively. Bee is simultaneously journeying to find her real mother, and also journeying privately within her own mind, finding new ways to become autonomous and individual. So, in one sense, the novel is about a real investigation, but in another sense, it's about Bee accepting her mother's example for how to be an adult.
And how does Bernadette demonstrate adulthood? With absolute freedom. When she becomes cloistered at home, both literally and figuratively, she breaks free without a moment's notice. She realizes that her professional life and her personal life are both infected by dysfunction, and instead of getting tangled up with the FBI's business with Marjuna, and instead of fighting things out with Elgin (who has a mistress), she just leaves. And where does she go? To the very place her daughter wanted to go in the beginning of the novel (another reason to take Bernadette as a symbol for Bee's burgeoning autonomy.
In the end, Bee is readmitted to her school, and she finds a missing letter from her mother that explains this all. This ironic twist is a reminder that Bernadette wanted to be found, but Bee's misbehavior made that process way more confusing and difficult than it needed to be. Bee will be adjusted to adult life when she combines her mother's artistic spirit (wildness and unpredictability) with her mother's attention to detail and discipline (she is not just an artist, she is an artistic architect, which means she has math degrees, so Bee has some catching up to do in terms of maturity and hard work).
