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I usually have someone's business card on me, so I'll stick that in the card slot so that I can head to dinner while stuff recharges or the room cools down (for places where the A/C is tied to the same switch). I have yet to see one that's anything more than a simple mechanical switch.

Ask for two card keys and leave one in the room.

At least at the one I stayed at once, you didn't need your room card in the slot, just a card. YMMV.

Though I wouldn't be surprised that the cleaning staff would "clean" it up if you weren't there. Might want to put out the Do-Not-Disturb sign too.

The one I ran into that did that had an outlet that would work when out of the room... in the bathroom, for a rechargeable shaver, presumably. So my laptop got plugged in there...