Hydrogen is perfectly stable until you put a match to it...
The amount of hydrogen produced during charging is Typically not enough to be concerned with unless the cart is stored in a totally enclosed area. Hydrogen being lighter than air will rise, so usually some kind of venting in the upper section of the building or area will provide a easy escape path for any that is produced. The bigger concern IMO is hydrogen sulfide which will cause sulphur dropout and can create sulphuric acid if sufficient moisture is in the room. If this gets into the ventilation system of the house this can cause damage to ductwork, and the evaporator core of the AC system which can cause heavy corrosion and degradation of the material of the core.