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. For overcharging and battery leakage we are looking for VOCs
When using liquid cooling for your batteries, then you have also to consider a Hydrogen H2 sensor. Leaks in such systems can result in Hydrogen and thus require discrete H2 sensing.