The most likely cause is water dripping through penetrations in the wall. I'd remove the faucet knobs and escutcheons as well as the tub spout and plug any holes that water could go through with caulk.
Also, it COULD just be condensation forming on the cold water pipe and dripping off.
I would open the wall sideways rather than up so that you can use a flashlight and mirror in the wall to tell where the leak is.
I can reach in and feel some water leaking, but only when the water is on.
what exactly do you mean?
A. you can feel water dripping on your hand only when the water shut off valves to the bathroom are open, or
B. you can feel water dripping on your hand only when the bathtub faucet is running, or
C. you can feel water dripping on your hand only when the shower is on.
If it's B, then it's not a leak in the supply piping, it's a leak from the faucet or downstream of the faucet. I'd use a flashlight and mirror to see if the water is coming in through the hole in the wall for the tub spout.