I had exactly the same problem on my 53reg Astra 2.2 sri. Twice!
Mechanics at the garage I took it to said the crack was either internal or that small it couldn't be seen.
Also been told by a VX mechanic, its a common problem especially on the 4th spark plug, and its more likely to cause problems the next time you replace the plugs rather than the time they were overtightened. This apparently is due to the crack opening up as you remove the spark plug, then when you replace it the threads don't line up and it makes it even worse.
Makes sense, as when I noticed water covering my 4th plug, I decided in my infinite wisdom to clear the water out by removing the pug and turning the engine over, then put the plugs back in. To my ultimate horror water started to leak out everywhere and almost drained the system due to the pressure as the engine was still a little warm.
My advice if you notice water covering your spark plugs on this engine, is DO NOT remove the plugs until you are home or ready to replace the head.
Real shame as the engine otherwise is one of the best ive ever had the pleasure of owning.
Apparently there is a SAAB head which is manufactured using a different process which is a lot stronger. I believe it is from the SAAB B207 which I think is the same as the GM Z20NET? Maybe someone more experienced could shed some light as this is apparently a much better head.