Are there any RF signals a car emits when the engine is running?
I know there’s the plug under the dashboard, or even just the DC outlet (“cigarette lighter”) that would provide a hardwired indication, but I’d like to find something that can sense the car being on without having to make a physical connection.
With all the electronics in there now (some cars have WiFi, right?) I’m hoping there is something, and the more widely available the better.
Thanks
Sure, the ignition system generates all kinds of RF.
Try using an AM radio set to a “dead spot.”
surprisingly little; there’s of course RF noise from all of the electronics but modern cars have rather stringent EMI limits so there’s not really much in the way of discrete signals escaping. You might be able to pick up Bluetooth communication if the infotainment is using it, or cellular communication if it has a modem. Or keyless entry communication as it “pings” for the key fob.
Some cars will have bluetooth or built in cell phones, but your most reliable indication is probably going to be the RF noise from the spark plugs.
Depending on the car, that may not be all that reliable. Many modern cars put out relatively low levels of RF, and there are some high performance spark plugs that put out so much RF that a car 20 yards in front of you might put more RF noise into your receiver than the car you are trying to measure, possibly giving you a false indication that the car you want to measure is running.
If you can tap into the car’s electrical system, the resting voltage of a car’s battery is somewhere around 12.4 to 12.6 volts or so depending on how well charged it is. This is going to jump up by at least a volt or so when the car is running, assuming that the car has a properly functioning alternator.
ignition systems don’t emit much RF anymore. the vast majority of engines today are coil-on-plug (no spark plug wires,) and the actual ignition spark is inside the cylinder where any RF energy is shunted away via the grounded engine block.
Sense from what distance? Close contact, underhood, five feet away, from Mars?
For most cars made in the 21st Century, the OBD-II port would give you indications that would show that the engine is running or not. Wireless Bluetooth OBD-II dongles are cheap and compatible with smartphones, tablets, or PCs/laptops with Bluetooth capability. The dongles can also be left in place even when not being monitored.
But that’s not entirely stock. You do have to buy a doohickey and plug it in under the dashboard.
That’s why I said it may not be all that reliable since modern cars put out relatively low levels of RF. 
RFI was why resistor type sparkplugs and wires, and other suppression components were introduced. Automobiles were causing lots of trouble with radio and television reception.