When the radiator is leaking and it's throwing out water
Most likely not hazardous. The "yellow brown spots" are probably just from rust, and that's perfectly normal. When antifreeze is used in a hot water circulating system, it's pretty much always a non-toxic form, and it's actually unusual .vs. just using plain water. 

In a system with iron components and no leaks, the oxygen in the water causes rust for a short time, and then stops rusting as the oxygen is used up. When the system leaks, more oxygen is available and more rust happens. But there is always rust in a heating system with iron components exposed to the water, and that's normal.