They're called Take Charge and Go EV Charging Hangers, and they look like the "Do Not Disturb" signs people post on hotel-room doorknobs. Designed for either dashboards, rear-view mirrors, or charging nozzles, EV drivers can place the placards in a direction where they show either the green side (which indicates an "opportunity" charger and indicates that it's OK to unplug) or the red side (which indicates "necessity" charging, or do not touch). And there's enough room to write on the red side with a dry-erase marker indicating what time charging should be done. If you're curious, we don't expect many people to admit to opportunity charging, either.
The placards, made from recycled paper card stock, sell for $4.99 a pop on eBay, though are now being promoted at half-price and can be ordered in quantities as large as 100 for as little as $1.25 each.
Way back in the Dark Ages (by plug-in vehicle standards) of 2012, Ford pushed the concept of plug-in etiquette to the forefront of at least some people's consciousness by making a placard electric-vehicle drivers could use to let other folks know when they'd be done recharging at a public station. Among the now commonly known rules were admonitions for conventional vehicles to not take up spots designated for plug-in vehicles (getting ICE'd), and for battery-electric vehicles to take priority over plug-in hybrids when it comes to recharging pecking order.