The sun produces visible and non-visible radiation. A significant amount is UV. Your cover may be opaque to visible light but allows some UV to pass. The UV will generate power.
If you want 0 solar radiation to pass through you need to block all radiation and it must be 'dark' colored.
Next question would be what make and model of meter are you using. Does it have a low current setting?
I have seen solar panels generate energy through inches of ice and snow ... why was I there? Because the energy was not adequate for the load but the panel was still active.
Linear Mode
