A microwave oven :
A microwave oven converts only part of its electrical input into microwave energy. An average consumer microwave oven consumes 1100 Watts of electricity in producing 700 W of microwave power, an efficiency of 64%
So within the cavity it is something like 700Watts
Blue tooth :
The Bluetooth specification targets power consumption of the device from a "hold" mode consuming 30 micro amps to the active transmitting range of 8-30 milliamps (or less than 1/10th of a watt).
100milliwatts top, from the link.
These numbers tell me
a) that the energy needed to cook a kilo of meat in 10 minutes or boil water in a cup in 1 minute is about 7000 times higher than the highest energy consumed by Bluetooth.
b)The microwave is a cavity resonator trapping the energy. Blue tooth is an antenna, radiating and necessarily following the 1/r^2 fall of intensity of radiated electromagnetic energy. At a distance from the ear(1cm) of even 20cm the impinging energy energy will fall 400 times. That is why people who are fastidious or overly scared, or who are always on their mobile use the "hands free" supplied with the instrument.
From Quora:
This is what happens when you run it without a load. At first, everything is fine because the cavity, the door, the tray (or shelf) absorb the excess energy. Things will start to get hot, but it's not critical yet. After a few minutes, something will start to get really hot and the excess energy? It starts reflecting back into the magnetron (the device that provides the microwave energy) and that starts to heat up too. If it is designed well enough, it will trip a thermal fuse before anything actually breaks. The thermal fuse can be resettable, but usually not. You will need to replace this fuse if the oven no longer works.
If the oven is designed poorly, you'll experience what is called "thermal runaway". At this point, some random location in the oven will begin to superheat. And by super heat, I mean hot enough to liquefy porcelain. This will continue until something finally gives out. It could be a breaker tripping, a standard fuse blowing, or in the worst case, the insulation on the wiring or the plastic components might literally burst into flames consuming everything flammable until the oven finally loses power.
Don't do this. Some ovens should not be operated when empty. Refer to the instruction manual for your oven. I would not ignore this instruction, issued by the FDA.
Also you would be wasting money, and I would guess there is some kind of limiter built in as hyportnex says, BUT it's not worth the risk.
But you could do this experiment instead, and eat the results.
Image source: Measure the speed of light using chocolate
Use a bar of chocolate to check that the speed of light is 300,000 km/s, rather than let all that energy go to waste.
Apologies if you know this already.
Measure the distance between the melted spots, after they have formed and then double it to get the wavelength of the microwave radiation. The wave frequency is around 2.45 gigahertz.
Velocity of light = wavelength x frequency
The distance between each melted spot should be around 6 cm.
6 x 2 x 2450000000 = 29400000000 cm/s, pretty close to the speed of light.
Best Answer
Just some rough numbers: say the oven produces ~1kW=60dBm RF power of which only 1mW=0dBm is allowed to leak out then the window's leakage is about -60dB. If your Bluetooth is radiating about 1mW =0dBm and your receiver has about -90dBm operating threshold meaning it will receive 0-60=-60dBm still having some 30dB (1,000X) margin above that value. A modern RF receiver is an amazingly sensitive device.