@belkrax
Yeah, the grenade was fake, which is why he left in the alley.
This was a really cool trick, since the stun grenade was real, so Yakou just assumed the M26 was real as well.
Baku could've just brought one grenade (an M26), but in that case, there would've been an actual chance for him to die if Yakou messed up the throwing (lol). And bringing only a stun grenade wouldn't have risked his life, which he needed to do in order to convince Yakou to fully ally with him.
Bringing one real and one fake grenade was really sly of him.