I just read a great thread on this topic that taught me a lot, but
I've got an unusual leaking shower situation I'd like advice on before
I take action.

This is a shower with one wall that is solid marble (think of the
stuff that used to be in old gymnasium showers) and two walls of
subway tile. The original floor and drain (which had octagonal tiles)
leaked water into the pantry ceiling below, so we tore all that out
and cut out the old drain and installed a new drain with Fernco
coupling and set the whole thing in a new bed of concrete with rubber
membrane underneath. On the concrete we set little square tiles in a
mortar bed. We also replaced three rows of subway tile on each of the
two walls, and herein lies my problem. The tile guy grouted the floor
and the new tile just fine, but where the new tile meets the original
tile, the grout has cracked. Obviously some expansion is happening
there, and I think this is where the water is getting downstairs.

I have tested the floor and the drain, and no water is manifesting. It
seems to be leaking through this junction in the tile walls.

Can I just use a good bath caulk, like DAP kitchen and bath adhesive
caulk, in this joint? Any BTDT tips on how I can prevent water
seepage?