engine spewing coolant

I replaced the ECT sensor, the thermostat and housing, and the radiator about 2-3 years ago. Started having intermittant overheats again about a week ago. No water seemed to be returning to the rad. After messing around with it, and coming up with nothing I could do myself, I took it down the street. They ran a pressure test and inspected it all. Flushed it, cleaned it, replaced the rad cap, thermostat, upper hose, bled it....etc...

Still got hot. Not overheat, but hot. Thought maybe the sensor was going bad again as when the fan was on, I was fine. So I jumped the sensor the fan would be on as long as there was power. (Better to kill a fan than to kill the engine, right?)

Everything was fine for about a day. While stopped I noticed green fluid coming out from underneath. Looked at the gauge and it was getting hot. Tried to drive it a bit to see if it would come back down, and when it didn't, I turned it off and parked it.

Popped the hood and there was coolant bubbling out of the back of the head. The rear of the engine compartment was covered.
My guess is the gasket or some seal I don't know about.

So, two things; 1) What the hell happened? and 2) Should the shop have found this when they did the pressure/leak test?

Oh, and later I had to carefully get it home, but it didn't overheat at all on the way.

Thanks for any help.

did the shop flow test rad or just pressure test??how many miles was the drive home??how many miles on engine??

They did pressure test on cooling system, and they said they did a head gasket leak test, but no mention of a flow test.

the drive home was fairly short, maybe 2 miles, but enough to let it get up to temp and stay there for about a song (4minutes or so).

And as for the engine, it has right at 323,000.