Heater core is like a mini radiator that’s used to HEAT the cabin of your car during the winter. Yes you are NOT running the heater…however water/antifreeze is still circulating through it. When you turn the heat on a door opens up that diverts the air over the heater core that will heat the air and pass it through into the cabin. The heater core and AC all share the same air ducts. So if the core is leaking you’ll smell it inside the cabin.
Get this fixed pronto. If it’s leaking enough you’ll notice the radiator to be low. It could also dump the fluid onto the carpet and floor of the car which then has to be cleaned up.
PS…Don’t do the $50 solution mentioned above. It’s NOT worth it.