Shops like that tend to have security cameras, as they get robbed a lot. John Korpan, the Ministry field operative up on the fire escape, knows roughly how much of a visual field most varieties of security camera in Canada can cover. This is why he waits until Tyler is outside of said visual field by one and a half of his own body's length to set off the minor explosive that he chucked into the storm sewer by the curb while Tyler was in the store.

The bad luck outcome will involve Tyler either darting back to the store or falling over on the spot. The good one will involve Tyler attempting to evade the sudden spew of yuk by ducking into the alley.