Here's a good close-up video of the exact model in use. This video suggests I am wrong in my post above about the automatic bolt position for this model on an empty magazine:
https://www.youtube.com/watch?v=qtardPylgL8You can see how the operator would be able to surmise, for safety purposes, whether the weapon is chambered - albeit not easily, because it would require examining the chamber from a right-hand perspective, though that could be achieved by the simple expedient of turning the gun to one side anti-clockwise.
Separately, in terms of surmising his magazine situation, I had believed with models of that vintage it was generally a case of 'count and shoot' until you 're-clip', but this video indicates to me that in fact the 525 Model does lock back the bolt in the open position, suggesting that an operator would know when to re-load the magazine.
Of course, it does not follow that the operator will always have an empty magazine in that situation and it doesn't change the logistics of a killing much, or the planning aspects for a sophisticated perpetrator.