The air bubble on the top of the pressurized surge tank is supposed to be there. Its spring-endowed cap may vent at its rated PSI, but it's supposed to vent air from the bubble. If it burps coolant the system was either overfilled or the car is overheating and boiling over.
In the old days, pre 1970s, the top of the radiator was the pressurized surge tank. It would burp out coolant the first time it got up to temp, replacing that volume with air, and that was it. The Chevy Vega needed all the surface area of its tiny radiator, so they invented the (non-pressurized) overflow tank (and vacuum/ siphon return system) around its inadequacies.
Then when cars got more aerodynamic, the top of the radiator was no longer the high point, and surge tanks became popular to help de-aerate and fill the system.
"Where does all the steam go?" Well there shouldn't be steam. If the cap lets a little air go at 15 psi there's still more pressurized air underneath it keeping the coolant from boiling. It doesn't reduce to zero psi. Aside from a little hysteresis it will keep on maintaining that pressure.
Your assumption seems to be that a cap venting (dramatically) is an emergency pressure relief, which it is, but it's also part of a feedback system that lets a little pressure out (due to normal thermal expansion) that you wouldn't even notice. If you look at the rubber flap on the bottom of the cap, it folds away from the metal. This is the check valve that keeps the cap from maintaining a vacuum, so as the car cools down, air finds its way back in.
To get "dramatic" steam, remove the cap on a very hot car. The pressure that was holding the coolant from boiling is suddenly released, the system flash-boils, and a lot of it comes out the rad cap scalding the hapless guy who removed it hot in defiance of all written instruction. A failure of a hose or anything else that holds pressure in can also cause this dramatic release.