I have the incanto and Im assuming its the same thing- a silver bumpy knob that operates the steam. Only mine doesnt have a pressure release in it at all! :o
Ive taken mine off a number of times when dismantling the machine and its a simple process of sliding it back on.
Are you sure it popped off with the pressure from the steam?
Unbelievable! I reckon- clear the steam wand with a pin (wear gloves) and then pop the knob back on.