The hand grenade is a cut weapon from Zombie Master, it is very scarce and powerful, it uses the same grenade model from HL2
It can be only obtained via-mods or in custom servers, and it still unkdown why it was cut, possibly because in the mod exists many exploding barrels, so it would be a bit useless.
The Soldier bodies in some original maps has hand grenades on his bandolier, but cannot be picked up.
This weapon is very common in custom servers.
Altough it is called as Hand Grenade in the files, it looks like a Flashbang instead of a normal hand grenade.
On the first picture of Zombie Master of the Steam Store page, if you have nice eyes, you can see that rotinaj is using a grenade.