buddy of mine got one for his wife and it went off in the rain
I don’t know about the brand your buddy has but that sounds like a one off. Humidity along with a bobbin that hasn’t been changed out over years will make them pop or even if they are kept in a humid below deck space.
The ones I use and others I have seen are all designed where the trigger is protected from rain water. Generally there’s an opening at the bottom so when a person goes into the drink, the water can enter the trigger area to inflate the vest.
There’s just too many variables to guess why that one went off in the rain.
I will say the first evening out with my three Float Techs we ran into a heavy rain storm. It was Wade Kuehl, his dad and I. The whole 10 miles we ran back to the dock I was expecting the hear each one inflate. Didn’t happen…whew!