The classic external head gasket leak is a slow drip usually on the back passenger side of the block where the head meets the block. I nursed that one along for quite a few thousand miles before doing the HG's.

How that works is a small break in the narrow section of gasket allows coolant out an air in. It usually is enough to notice the slow drop in your overflow reservoir. Not paying attention to the coolant level there will eventually lead to your situation. You can put off the repair by staying eagle eye on top of your coolant level.

Use a small mirror and the pressure test gizmo to see the the leak at the back of the engine... Or just take a look with a mirror after the engine is hot and you have shut it down. The internal coolant system pressure will bleed off through the leak if you have one.