Neither NiCad nor NiMh should leak. However, if they were ever exposed to a high temperature or were overcharged at a rate that raised the internal temperature to the point where the gas pressure generated inside the battery blew the seal, then it could leak electrolyte with resulting corrosion and/or degradation of the surroundings. These batteries have a plastic seal at the positive end that can be deformed at high temperatures or internal pressure. This is a safety factor to keep them from violently exploding when improperly charged. An added problem, if the batteries are NiCad, is the generation of black wire disease when that electrolyte leaks. See
http://www.hangtimes.com/redsbatteryclinic.html for some good info on batteries including "black wire disease".