Right before a complete thermal runaway process, failing li-ion batteries will release gases due to the internal pressure blowing up the battery enclosure. This is stage 2.
In this phase you have a few seconds to a few minutes at most. You already have at least one or more batteries having a catastrophic failure. You are trying to limit the propagation of a thermal runaway.
At InfraSensing we invented the daisy chained off-gas sensor. We introduced offgas sensing not just by electrolytes but also by looking at CO2 gas. In most instances of first venting or off gas events, CO2 is the key and most stable gas.
When using liquid cooling for your batteries, then you have also to consider a Hydrogen H2 sensor. Battery leaks can happen in that case at normal temperatures and will release H2 before any other gas.