Shower Drain Issue

Hey guys. New here, but I'm having an issue with my one shower drain and I thought yunz would be able to help. We have two bathrooms, one I recently remodeled and have been using regularly. The master bathroom is in need of a remodel, so we haven't been using the shower in there (only using as a half bath) since I completed the reno in the main bathroom. Since we *stopped* using the shower in the master bathroom, we're getting a noticeable vent-gas smell emanating from the shower in the master bathroom. We've occasionally run water down the thing (1x/wk) to make sure that we have water in the trap to maintain a seal, but the odor keeps returning... a little too quickly for the water in the trap to evaporate. I did a little exploration and found this:



At first glance I thought I had a galvanized trap that was at end-of-life (my house was built in '65), but after putting a small hole in my ceiling, the trap is cast, and transitions to the main soil line, which is copper. The bizarre thing (to me) is that I have all this corrosion that looks just like what comes off galvanzied pipe when it is at the end of life... Could my shower drain be galvanized? Did they ever make such a thing?

I plan to have a pro come out and fix this, but I'd like to have some information in my back pocket if possible when he or she comes out. I'd prefer to *not* have to rip out the entire bathroom. Is this something that can be fixed? Thanks very much for all the help in advance.

From the angle the picture was taken, and the limited lighting, I have no idea what I am looking at.

Looks like a 3" Cast P Trap tied into the copper line. There appears to be something going on there but really hard to see what from such a far distance.

The shower drain - can it be snaked? It looks like the fitting is gone and I would bet leaks.

What happens when you run water down this line? Have you tried running the shower for twenty and watching this?

JW