That was easy enough to parse even without Thor adding the detail. From the previous parts of the story, Loki knew Thor had sent him to wake Surtur using his crown and the Eternal Flame.

In Odin's vault, where the Tesseract was also kept.

Of course he'd taken it in passing. Despite what Thor probably thought, he couldn't even consider taking it to have been a mistake. It was a bargaining chip, and a good one... in theory, anyway.

Was there perhaps a hole to exploit, when he got to that point? It would bear thought. Later. When he could think properly again. "I hope I at least tried to make it difficult for him."