Smoke generation is a sure sign of thermal runaway, as well as a precursor to ignition.
Current detection methods, including lower explosive limit (LEL) and smoke detection, will only alert you after thermal runaway has occurred. Do not rely on the current technology utilized by fire departments or even the majority of current fixed systems to provide advance notification of thermal runaway.
Once ignition occurs, the resulting fire or explosion can heat and compromise adjacent battery banks. In recent utility-scale incidents, this type of propagation has consumed multiple battery banks within a BESS enclosure.