What might be happening here is that water is being driven behind the
siding above the window and running behind the window fins and into
your house that way.

Ok, but the drywall BELOW the corner of the window frame was registering as
wet or excessively moist according to the moisture detector, yet the drywall
above the top of the frame, as well as to the side of the window frame,
seemed to be dry according to the moisture detector. Was wondering if my
only finding moist drywall below the bottom corner of the window frame (as
opposed to above the top or the frame or next to the side of the frame)
indicate that it's not likely to be a siding issue?

The wallpaper itself was dry and no indication of water on the inner sill
though. Is it possible that water got beyond the sash seals and then
migrated directly behind the drywall from there rather than dripping onto
the inner sill?

Thanks,

J.