Books in Piccadilly Street series

There is a hidden treasure in the grand old mansion on Piccadilly Street, in a place called London, but not the real London of English fame. There's also a lot of mystery and a murder that's been unsolved for decades. But it's the treasure that captures Mary's interest. Mary lives in this house along with her family, her Brownie friends and a ghost. When the ghost reveals her secret about the hidden treasure, there's no stopping Mary, her Brownie friends, or her enemies from searching for this treasure. Why the intrigue? Apparently there's a little bit of magic connected to this treasure. And so the adventure begins. Who will find the treasure first?