The last time I used them was earlier this year, around February. I opened the visor before and after the trip (but not slid out the visor lights), to put a garage clicker on it. The light was definitely not open.
So I can easily believe – and in fact now think it's the most likely explanation – that the renter slid the cover open and the light didn't come on. Without the light on, there's no reason to even think the slider should be closed. Someone unfamiliar with this mechanism might not even realize that when everything is working, the slider works as an on/off switch.