I've got it now, on a used "for parts" SunPower branded SB 7000US I just bought.
I wired it up, it was "waiting" to qualify the grid, then from a distance I heard what I thought might be relays clacking closed. But too loud for this model. Besides, it was more of a loud "Splat". I lifted off the lid, observed the messages, heard another "Splat" and saw sparks fly, I think from vicinity of display board lower right.
Disconnected for now.