Te mall in question is the rideau centre, and the shooting was "near" it according to police reports, which seems to have been dropped by media. My guess would be either on Mackenzie King bridge(which is attached to the mall and also attached to a military building). It is the hub station of downtown transit as well. There is often at least 50 or so people standing On said bridge, at 9-10 am likely quite a bit more.
The other side of the mall is rideau street, which connects to where parliament hill is, and is also quite close to the US embassy and many other federal buildings so who knows.