leak inside shower wall -- help!
 

I recently noticed the water pressure was lower than before for our shower/bath. The shower backs to a linen closet with a small access opening way down low near the bottom (too low to access the fixtures.) Anyway, I can reach in and feel some water leaking, but only when the water is on.

Not sure what to do next, other than open the wall a bit higher near the bath-level fixtures. Any advice? (Beware I'm clureless when it comes to plumber terminology.)

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.

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.

Do you have a diverter spout?
Does it leak worse when using the shower?

What brand/model valve do you have?