I find it unusual that machines don't have vented battery compartments even with nicads.
The only thermal runaway I experienced, in a nicad-batteried Bell 204, was noticed first by another machine in the circuit with us at night (both in the pad at the same time) by the 'steam' coming from the battery vent down near the tailboom/cabin junction.
Aren't nicad installations set up with vents, in some cases through telltale jar arrangements with treated material in them that changes colour in the presence of battery fumes?
Maybe not on all types.