No. The chances of this being any sort of issue are effectively zero.
It is important to distinguish between the events of charging the battery and discharging the battery. With improper charging (ie, a cheap or malfunctioning battery charger) the battery can be made to vent (its not common, but it is possible). However, venting does not happen during discharging. The box only uses the battery in discharge mode. Therefore, there is no risk.
Assuming that somehow the battery did vent, in considering risk it is also worthwhile to consider what kind of gasses are involved and how much might be generated. From the chemistry, the gas would only be H2 (hydrogen) and its quantity would be tiny (a few cubic centimeters at most). Both the gas and its amount are biologically harmless. Again, no risk. For people who work with batteries in general, these points are considered to be common knowledge.
The hole in the back of the unit is a construction hole. It connects to the battery compartment but does not extend through to the vapor chamber. As such, it does not provide any form of direct coupling between the battery compartment and the vapor chamber. To ensure that there could be no doubt of this, the seal on the connector rod has even been tested to 5 PSI. Therefore, even if the battery did vent (see 1 and 2 above) it would not be available for inhalation, but would go instead in the opposite direction away from the user. Again, no risk.
In the interests of safety, however, it must also be reported that some chargers do not do as good a job detecting when a battery is already charged. If people put an already mostly charged battery into a fast charger, and that fast charger is not smart enough to detect this condition, it could potentially overcharge the battery causing it to get hot, and in particularly bad cases, to vent, leaving a white residue (not the end of the world). For some chargers, it can also be rather bad to put the battery in backwards. (Don't try it!) High quality chargers generally handle both of these conditions gracefully.