If improper filling is completely ruled out, then based on the rest of the information I would start investigating a cracked cylinder head. Here’s why;
A radiator cap is supposed to hold back a certain amount of pressure (this amount is usually printed on the cap) but will vent excess pressure to the overflow container to allow for the expansion of the coolant as it’s heated. If unheated coolant is being vented then either the cap is bad (like Gary T said) or cylinder compression is pressurizing the block past the limits of the cap (also what Gary T said). A cold engine with a good cap should not vent coolant.
Also, while the coolant is cold the thermostat is closed and holding back the coolant. This allows the engine temperature to rise to normal operating temps before the coolant is circulated. Some people remove the thermostat altogether, not realizing that it will take even longer for the engine to warm up and start running without the choke. I mention the thermostat because it operates similiarly to the radiator cap in that it is normally closed and holding back the coolant but, whereas a the cap will vent coolant based on the pressure, the thermostat will open based on the temperature of the coolant…unless the pressure behind the thermostat is too great for the spring which then will mechanically force the thermostat open allowing the coolant to flow past. If you look under the cap of a cold engine when it’s first started, you should see the coolant just sitting there waiting for the the thermostat to open.
Look under the cap like Gary T said and if the engine is cold yet the coolant is flowing you either have a bad thermostat or cylinder pressure leaking into the coolant system is forcing the thermostat open.
Now, I said cracked cylinder head and not a blown head gasket, why? Because a blown head gasket will allow the vacuum in the cylinder on the intake stroke to draw coolant into the cylinder to be vaporized and exhausted as white smoke. Without the white smoke (really steam) the crack in the head would have to be in a part of the head isolated from the cylinder while it’s on the intake stroke, namely in the area behind the exhaust valve (on the stem side of the valve). The exhaust valve will be closed during compression and combustion and the pressures of each would not be forced through the crack, but when the exhaust valve is opened the pressure meets the crack. The piston coming up, pressurizing the cylinder to exhaust the burnt gases could force excess pressure through the crack, into the coolant system, past the thermostat and finally through the radiator cap.
A blown head gasket can cause the same scenario but, without the white steam, it seems unlikely.
If it is a crack in the area I described it will not show up as such in a compression check since the proper way of performing the test calls for both valves to be closed. So, if all other observations point to a crack or blown gasket but the compression test is fine, it’s probably a crack in the exhaust valve bowl.
Too put all this in one paragraph; look for air bubbling through the coolant like Gary T said and if you see it then you have a cylinder head problem. If you see absolutely no white steam in your exhaust it is likely a crack and not a gasket. Perform a compression test, if it checks out, it’s a crack and not the gasket.