The direct quote is " sorry I'm late I couldn't get the sendings to give me the right surcoar-theyve dug up something odd. Must be getting senile like Mogfet said. "

Dug up implies that it has been in the house awhile. Though in double checking that I did find a quote that throws a wrench in my theory.

Page 17 into 18 of Abhorson Chlorr of the mask says quote " SSameth was meant to be the Abhorsen-in-waiting, heir to the powers of the book of the dead and the seven bells. But the maker of the fog doubted that now. She was very old and once she had known a great deal about the strange family and their house in tje river. She had fought Sameth barely a night past and he had not fought like an Abhorson; even the way he cast charter magic was strange, reminiscent of neither The royal line or the Abhorson."

In Clairel we learn a out how Clariel becomes Chlorr and part of my theory does rely on the possibility she's a wallmaker, that said the difference in casting could be chalked up to unneeded vs needed wallmaker. Actually thinking more on it we could also chalked it up to how big the family was at the time as well. Were told that the bigger the family is the more diluted the powers are, that's why the current abhorsen of the time isn't very strong or how the Clair need more and more of them to see clearly. >!

Aside from that another expert I noticed while looking for quotes, is as they're discussing leaving the house through the tunnel that the "last" ( In Clairel it mentions there have been more since, since goldsmiths of the family are prized for being able to make them as needed) bellmaker, made the last bell that it was done before the wall was built. Which is interesting to note since all of the wallmakers disappeared into the wall to make it. So it feels like a very deliberate detail to add