Clearly Graendal didn't pay a ton of attention to Shara or she would have discovered Demandred was there. But items such as Sakarnen tend to create mythology around them ... like the Crystal Throne ... so if the rulers had some fabulous item in their possession she would have gotten some hint of it from either Jain Farstrider or the Sh'botai themselves.

Items of the Power tend to draw attention, even from people who cannot Channel ... I am more inclined to believe the Ayyad kept it hidden, perhaps not even fully understanding it's power themselves since the male Ayyad are sequestered and little more than animals. given what we know of the prophesies I could seek Sakarnen playing a role there, a nice surprise for Demandred. Although I have to say I found the whole thing to be rather contrived ... Demandred just happens to end up with one of the only sa'angreal more powerful than Callandor??

Which makes me wonder why it's more powerful. If it was built first why would the makers of Callandor make it less powerful? If it was created after how did they pull off a non-flawed sa'angreal of such power? Given that Lanfear was aware of it I imagine it was built before Callandor ... of course you have to wonder why the hell they bothered with making Callandor when the CK were in construction. Blah blah blah the Wheel needed Callandor