pull your fan shroud off and look around, all the coolant hoses (bypass, lower radiator, overflow line, etc) are in the general area and are all rubber, rubber deteriorates and can crack and leak, if its passenger side up near the block I'd suspect something like the water pump/gasket, or where the lower radiator hose connects to the waterpump, perhaps the hose is just old and sprung a leak, it could also be something like the thermostat housing. while you're there looking check out the hoses and just see if any of them look old. even if they're not leaking it would be worth replacing them.
I would doubt its a intake or head gasket, when those fail they tend to fail and leak coolant or oil internally somewhere, not to say it COULDNT happen, but IME its unlikely that it would just start spraying coolant. I'd put my money on A) hose leaking somewhere, B) waterpump gasket, C) thermostat housing, or D) maybe a freeze plug.
there's also a chance that the radiator plastic ends have a small crack somewhere that is only opening up when under pressure, and since the steam is coming from behind the alternator, it could be something like it is spraying a little coolant onto the exhaust manifolds or something. thats not the most likely scenario either, but possible.