No, for some reason it's not there before Kid reads the letter.
And it could be that Magus didn't know of Belthasar's plans, but even Lucca and Robo were involved, and Lucca even died. I highly doubt Magus didn't notice anything about that.
Or maybe he's living in 12,000 BC. At the end of Trigger it states Magus is looking for Schala across time, but how the hell can Magus time travel by himself without the Epoch?