Perhaps I should have been more specific. A missing caliper bolt will indeed make an enormous bang when the brakes are applied. This is the outward effect of the caliper pivoting on the remaining bolt and being slammed inward into the hub section of the rotor or swinging outward and knocking on whatever limiting factor it runs into first. Long term effects of this scenario include damaging the pads, scoring the rotors, or most likely damaging the caliper slides. Other effects can include severe damage to body work, bumpers, glass and loved ones as the result of acute loss of control and immediate impact. When in doubt, please have a trained professional physically inspect all four corners
Regardless, if the bolt is missing you will need to replace ASAP. And you MUST use a grade 8 bolt. No stainless or cheap stuff from Home Chepot. It's been a while, but I *think* the bolt is a 10mmx1.25 pitch, which I remember being a non-standard pitch for that size bolt. Best to bring a bolt with you and match it up.
Also, Rob's comments on brake fluid on the linings from leaking wheel cylinder and broken spring are salient and worth investigating.