Generally speaking, Muriatic Acid is worse than chlorinating liquid in terms of damage from gas leakage. Both bottles have caps that are intentionally designed to vent fumes when the pressure builds up. This is to prevent bursting of the bottle. A defective cap could leak continuously even with no pressure buildup.

Muriatic Acid stored in garages is known to rust bicycles and other metal nearby. If you go to a hardware store you will likely notice that metal shelves with MA have rust stains. So your theory is entirely plausible especially in an area with relatively poor ventilation (i.e. no wind regularly blowing through the area). The MA fumes are very destructive. I'm sorry this happened to you.

I store my MA in a pool box outside. There's nothing in the box I would be very sorry to lose and the box is open (with netting) on the bottom so gets some (but not much) ventilation. It's not ideal, but it works for me. I would not store MA in a pool shed since the heater and pump could get damaged. Even chlorinating liquid can be a little risky stored there, but is better than MA. If you have an open milk carton crate and some shady area of your property (say, behind the pool house, but not near any vent) then you can store MA and chlorine separately in such crates and the outdoor air circulation plus relative isolation should keep everything in good shape.

Richard