First, complete the "Historic Pioneer Square: Fire & Underground" Adventure Lab. When you have correctly entered codes to certain questions, you will see information in the Location Completion Messages to help you find this Bonus geocache.
Two main themes of the AL are
1) that the Great Fire of June 6, 1889 destroyed all wooden structures within the District, and
2) that rebuilding used beautiful stone architectures 1-2 stories above what became a disused Underground city.
You need not go to the above given coordinates which, though bogus, is blazingly on theme. The coordinates are for the Headquarters Seattle Fire Department (Truck Co. No. 1 - Water Tower No. 1 - Squad Co. No. 2 - Engine Co. No. 10) which is currently responsible for any fires within the Historic Pioneer Square district.
The Final coordinates and hints given to you in the AL are likewise on theme, underfoot. You do not have to enter any buildings, or ascend or descend, to locate the Bonus geocache. Please remain on the public sidewalk. Watch out for muggles as the entire area is a popular place to be.
FTF Honor goes to mc3cats!