Hmmm...I'd say that the five characters who are (going to be) in the cafe probably aren't close enough to feel, hear, or see the bombs (I don't know why Mikami would choose to go to a place that close to HOMURA's meeting place)

But I'd also say it probably won't take any of them very long to find out about it. Either through the other customers in the cafe freaking out or their own clansmen informing them in some way.