It is a very long time since I worked on Typhoon, prior to it being in service. My memory is very sketchy but I can remember a few things.
Jettison
Two types: Emergency Jettison (EJ) and Selective Jettison (SJ)
EJ will dump everything that is allowable, by pressing the EJ button.
SJ will blow off the group of allowable stores previously selected by the pilot, by pressing the SJ button.
The system knows what stores are fitted and what stores cannot be jettisoned.
I have a vague recollection that wing mounted missiles could not be jettisoned because they were rail mounted, as mentioned a bit earlier in this thread.
In any case the EJ/SJ buttons are hard to hit accidentally, as you would expect; the last thing you want is to dump all your stores accidentally (including external tanks). They were mounted on the left panelling, not on the HOTAS, so hard to press accidentally.
The Late Arm and firing trigger are on the stick.
The Telegraph has stated that the missile was fired:
"A statement from the Spanish Ministry of Defence confirmed that a missile had been accidentally fired by one of their Eurofighter jets".
That there was so much concern about the missile hitting other airborne aircraft also suggests that it was fired rather than jettisoned.
Reports indicate that the likely location of the missile is ten's of kilometres from where it was released, again suggesting a fired missile rather than jettisoned.