Anti freeze used for hydronic systems should be checked annually for PH and strength. Anti freeze looses strength over time and excessivly high boiler temperatures can degrade it even faster. PH is a concern because it can cause problems with seals etc on valves and controls. As for it leaking out, if the system is tight and there are no leaks it will stay in there and maintain pressure for many years. I always disconnect the automatic water feeder or don't install one on an anti freezed system. It is true that anti freeze will find small leaks that water would otherwise not and unlike water, antifreeze will not "rust up" the leak.