You know the trout is bad when the raccoons leave it alone
I made some home made vinegar from a portion of homemade wine. After several years (8?) of living in a basement topping off the bottles in the kitchen every few months with great results/flavour the acid seemed to be getting perceptibly lower. Taking a sample to work revealed less than 1% acid, meaning it had lost 9+% since original testing.
Don’t know where it went, unless someone was drawing off the jug and replacing it with water. No real explanation.