I'm still thinking gremlins. Get yourself a good gremlin-proof safe. They're not cheap, but they're worth it in juice savings.
On a more serious note, juice doesn't easily evaporate. You can steep juice by leaving the lid off the bottle and after months the level hasn't changed at all. So it's not that. As donnah said, the juice in the tank is equalizing with the juice in the carto, and the tank level goes down. Once it's equalized, it won't go down further.
Or, you know, gremlins.