  Every quasar (QSO) spectrum contains absorption-line signatures from the
interstellar medium, disk-halo interface, and circumgalactic medium (CGM) of
the Milky Way (MW). We analyze Hubble Space Telescope/Cosmic Origins
Spectrograph (COS) spectra of 132 QSOs to study the significance and origin of
SiIV absorption at $|v_{\rm LSR}|\leq100$ km/s in the Galactic halo. The gas in
the north predominantly falls in at $-50\lesssim v_{\rm LSR}\lesssim 0$ km/s,
whereas in the south, no such pattern is observed. The SiIV column density has
an average and a standard deviation of $\langle N_{\rm
SiIV}\rangle=(3.8\pm1.4)\times10^{13}$ cm$^{-2}$. At $|b|\gtrsim 30$ degree,
$N_{\rm SiIV}$ does not significantly correlate with $b$, which cannot be
explained by a commonly adopted flat-slab geometry. We propose a two-component
model to reconstruct the $N_{\rm SiIV}$-$b$ distribution: a plane-parallel
component $N_{\rm DH}^{\perp}$ to account for the MW's disk-halo interface and
a global component $N_{\rm G}$ to reproduce the weak dependence on $b$. We find
$N_{\rm DH}^{\perp}=1.3^{+4.7}_{-0.7}\times10^{12}$ cm$^{-2}$ and $N_{\rm
G}=(3.4\pm0.3)\times10^{13}$ cm$^{-2}$ on the basis of Bayesian analyses and
block bootstrapping. The global component is most likely to have a Galactic
origin, although its exact location is uncertain. If it were associated with
the MW's CGM, we would find $M_{\rm gas, all}\gtrsim4.7\times10^9\ M_{\odot}
(\frac{C_f}{1})(\frac{R}{75\ {\rm kpc}})^2 (\frac{f_{\rm
SiIV}}{0.3})^{-1}(\frac{Z}{0.3\ Z_{\odot}})^{-1}$ for the cool gas at all
velocities in the Galactic halo. Our analyses show that there is likely a
considerable amount of gas at $|v_{\rm LSR}|\leq100$ km s$^{-1}$ hidden in the
MW's CGM. Along with this work, we make our QSO dataset publicly available as
the COS Quasar Database for Galactic Absorption Lines (COS-GAL).
