A deer hit in the ribs will make a solid WHACK sound.

A gut shot deer will make that hollow watermelonimpact sound.

My guess you missed the first deer and you gut shot the second.

As far as a deer's reaction to the shot, you'll have to teach yourself to pay closer attention to how they react than you did while you were rifle hunting. Watch their tails,is the deerhunched over, are they at a dead sprint or are they only half running away or walking, are they bounding erratically from side to side as they run, etc... How the act at the shot and immediately after can give you a good indication as to where you hit the deer.

A good book for you to pick up and read multiple times would be Finding Wounded Deer by John Trout Jr.