It doesn't take water to cause corrosion in chrome plated steel parts. The electroplating chemicals are highly corrosive and need to be flushed out and/or neutralized. That didn't/doesn't always happen with bicycles. It can take years for the steel to corrode through.
Here's an example corrosion from the inside out with an old chrome plated fork blade. Does that hole look familiar?
Occasionally I hang out a a local frame builder's shop. He has a lot of examples frames rusting out from the inside. One heart breaker is an early 70's Colnago with playing card decals. It's red and just my size but sadly the tubes are Swiss cheesed (and it wasn't even plated).
I have a mid 60's bike that's chrome plated under a spectacular paint job. It too has "rain stick" seat stays. I got it from Switzerland about 10 years ago and it sat partially assembled for several years. As I was rotating the frame on the bike stand to work on it I heard the telltale sounds so I pulled the BB to check out the chain stays. They were full of large rust flakes. Unfortunately the bike will remain a wall hanger.
A friend picked up an old uber light weight lugless French constructeur frame at a bike swap. When he got it home he noticed pitting in the fork blades and seat stays. Stage IV cancer!
As someone pointed out the OP's frame is going to be expensive to repair. He can maybe get by with a new rear triangle if the rest of the frame is worth keeping and then save some money by not re-plating it. If the forks are questionable then find a nice place to hang it.
@
diomekes et al...
In reference to vent holes in the tubes, the main purpose is to allow gasses to escape while brazing. Without the holes, the heated expanding atmosphere in the tubes can push the molten brazing material out of the joint. On a lugged frame, it's mainly the seat stays, fork blades plus the seat and or head tube that have vent holes. The seat and or head tubes are drilled to provide an escape for gasses in the top tube during brazing.
Grape jelly or marmalade on that frame sir? Sorry...
verktyg
