I know it was a joke, but some readers will actually think you were serious, so allow me to clarify.
Firstly, how many times have hand granade pins popped/fallen/been pulled out accidently (in real life, not in the movies) - given the hundreds of thousands of troops deployed in the field over, say, the last two decades you'd expect a few if it was a problem. How many? None.
Secondly, what about the direct effects - the wearer or carrier of said hand grenade isn't going to like it going off, especially in company (as soldiers normally are) and in a vehicle (with petrol/diesel tank) or next to other more dangerous munitions (which occurs frequently). So a fuel cell hardly registers on the risk scale.
Thirdly - have you seen methanol burn? Frankly, it's the kind of stuff you can stamp out with your foot if the puddle is small enough - I've seen worse flambe's.
So realistically (as opposed to books, films and user-submitted jokes), there's no problem.
Black Helicopters - because fuel cells and batteries would both be safe to transport by air.