= Solution
Spin-polarized injection produces different spin-resolved <electrochemical potentials> in Cu, $\mu_s=\mu_\uparrow-\mu_\downarrow$. Their imbalance diffuses beyond the charge-current path. Oppositely directed spin-resolved currents can cancel in net charge while transporting spin.
The second Py contact is a spin-selective voltage probe. In a weakly invasive model,
$$
V_{\rm NL}-V_0\simeq\frac{P_{\rm det}}{2e}\mu_s(L),\qquad
\mu_s(L)\propto P_{\rm inj}I\,e^{-L/\lambda_s}.
$$
Its open-circuit potential adjusts until the total detector charge current vanishes. That potential changes when the detector <magnetization> reverses relative to the injected spins. \b[Zero detector charge current does not imply zero spin imbalance or zero measured voltage.]
Reverse injector current and each electrode's alignment to test the predicted signal symmetries. Remove spin-independent thermoelectric offsets and stray-field Hall contributions. A nonzero voltage alone would not prove <spin accumulation>.<Spin injection and spin accumulation in all-metal mesoscopic spin valves by F. J. Jedema and others>[2003, nonlocal detection and separation from contact-related magnetoresistance]{ref}
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