Good question TB. I think there are several factors that come into play. The shots would probably be heard at the compound if one was expecting to hear them. But Connie and Tony were probably inside as was Tom. The guards? They may have heard them and put 2 and 2 together. However, the sounds of gunfire can be affected by several environmental factors.

I'm sure Michael could arrange for "select authorities" to investigate Fredo's accident.