I should probably start by saying that until today, I'd never heard of KI Starch Paper, but the problem intrigued me, so I looked it up!
From what I gather the paper changes colour in the presence of oxidising agents (like peroxides) but also others like free Chlorine and Iodine.
Given that your waste container contains KI and HCL, could this be generating free Iodine perhaps?
Just a thought...