Microwave question

Not much, if anything at all.

A microwave's heating action depends on having an object which will heat up through "dielectric" activity - that is effectively the rapid back and forth movement of small "polar" molecules within a substance to generate heat. In the case of a thin label, the density of these polar molecules isn't sufficient to really do much. If you had inserted a damp piece of wood though, you'd notice the moisture boiling off the wood before it starts to brown in patches and starts smoking as it dries out, but it's unlikely to burst into flames as long as there isn't a spark.

Some labels though have metal within the printing foil and these aren't supposed to be used within a microwave, but if you do you will often see tiny fractals or spots where that metal reacts with the microwave and "pops" where a localised "short-circuit" occurs. Nothing much to wory about, but don't put foil into a microwave or you'll get a concentration of energy and then a spark as that accumulated voltage is disapated into the microwave's metal shell.