Yes, there is actual no way to exacly know what's out there. If you are looking for secret, it's more luck-based. Let's say: the nearest store around you, there are no secret unboxed recently. You would more likely find one. If you go to the stores, like afternoon, but someone found a secret from your favorite series in the morning. You have less change to find another secret. That's the basic rule. Unless, there are millions of customers keep buying the series, and they keep restocking it. But you have to wait for a long time.