Assuming you have your standard Voldemort fights the Muggle World scenario, how could Muggles, assuming they know that wizards can turn invisible, go about detecting them?
I have some ideas, but I'd like to hear your input and suggestions.
Neither Invisibility Cloaks nor Disillusionment Charms do anything to block scent or sound, and most wizards would probably not think to mask these signs. A well-trained dog might be able to smell or hear an invisible intruder and bark to alert its handlers of the invisible wizard's presence. Unfortunately, this doesn't let the Muggles see the wizard; it only lets them know they might be in the area. Dogs could also be easily distracted.
At least this method would be cheap and easy, and should provide a minimum of detection.
Invisibility Cloaks and Disillusionment Charms do nothing to hide the wizard's body heat, and having wizards charm themselves to be the same temperature as their surroundings sounds like something they'd never think to do. So even if a wizard is invisible in the visible light spectrum, he can still be seen through an infrared camera as a bright red-yellow figure against a blue-ish background. If a soldier has goggle-mounted infrared imaging, he can see the invisible wizard through IR and shoot at him. This sounds to me like the most precise method of invisible-wizard detection, but also very expensive and uncommon, as IR imaging isn't cheap.
The Disillusionment Charm is a sort of active camouflage rather than true invisibility, which means that someone who is using the charm will still cast a shadow. Seeing a disembodied shadow walking around would tip off anyone who notices that there is an invisible wizard there. Unfortunately, this method depends on Muggles being very observant, and it is useless at night or against wizards using Invisibility Cloaks. Still, at least it's something anybody can do on their own.
Very thin tripwires attached to directional mines or guns should take care of any invisible wizard who isn't watching his step. The triggering mechanisms can also be very sensitive in order to also go off in case the wires lose tension, which would happen if the intruding wizard cuts or vanishes the wires. Unfortunately, this method sounds rather dangerous and hard to set up, so it would probably be limited to heavily protected installations guarding, say, the Royal Family.
Funnily enough, US and British soldiers in Iraq have been using Silly String to detect thin tripwires, in which the settles on the wire, revealing them. The same thing could be done against an invisible wizard. This method is limited by the very short spray range of the aerosol can, the Muggle wielder being alert enough to suspect there may be an invisible intruder to spray at in the first place, and the invisible intruder being very cooperative by standing close and still enough to get sprayed. At least Silly String is comparatively cheap, and it's better than nothing.
Sprinklers, Hoses, and Miscellaneous Liquids
Applying the same logic as the Silly String, water from sprinklers or hoses would splash off an invisible wizard and soak his clothing. Even better, bright paint could be sprayed, making the wizard much more visible if it were to get on him. Any continuous sprinkler-based wizard detection however, would require an awful lot of water or paint, and it can only be used at fixed locations. Manned hoses attached to fire engines could provide some mobile detection, but like many of these methods, it depends on the wielders being alert and spraying anything suspicious. Water balloons filled with paint are another possibility.
Those are some methods I've thought of off the top of my head. Does anyone have any other ideas?
Tim the Enchanter