Something Is Up There
Toronto has a well-established raccoon and squirrel population and they are very good at the roof-to-soffit junction.
Short answer
The entry points are the soffit-to-fascia joint, the roof edge where the soffit meets the roof, a corner where two soffit runs meet, a damaged or unscreened vent, and the gap at a plumbing or wire penetration. The animal has to be out before the gap is closed, and in Ontario there are rules about how and when that can be done.
Where they get in
The soffit-to-fascia joint, particularly where the fascia has rotted and the panel no longer sits tight.
The roof edge at the soffit return, which on a Toronto house with a shallow overhang is a small gap that gets widened.
Corners where two soffit runs meet, which are frequently the weakest point.
Soffit vents with no screening or with damaged screening.
Gaps at plumbing stacks, wires and service penetrations.
And the roof-to-wall junction on an addition or a dormer, where the detail is frequently imperfect.
Rot is the enabler
A squirrel can widen a small gap and a raccoon can pull apart a soffit panel, but both of them start where something is already weak. Rotted fascia, a loose panel, a corner that has opened. Which means the soffit and fascia condition is the animal prevention as much as the eavestrough performance.
Getting them out first
Never seal a gap without being certain the animal has left. A sealed-in animal dies in the soffit and becomes a far worse problem.
Raccoons in Ontario are frequently nesting with young in spring and early summer, and sealing a mother out from her young is both inhumane and ineffective — she will make a new hole.
A wildlife removal service handles this properly, typically with a one-way door that lets the animal out and not back in.
There are Ontario rules about trapping and relocating wildlife, including distance limits, which a licensed operator will know.
Expect the removal and the repair to be two visits, with the one-way door in place between them.
And expect a return attempt, because the animal knows the location.
The repair
Replace rotted fascia and soffit rather than patching, because an animal returns to a weak point.
Close the gap with material that cannot be chewed or pulled — heavy gauge metal or screening rather than foam or plastic.
Screen every soffit vent with metal mesh, which also keeps the venting working.
Seal the penetrations and the corners properly.
Clean up and remove nesting material and droppings from the attic, which is a health matter as well as an odour one, and which needs the right protective equipment.
And trim the branches giving roof access, which is the measure that reduces the return rate more than anything else.
Common questions
Can I just seal the hole?
Not until you are certain the animal has left. A sealed-in animal is a far worse problem.
Where do they usually get in?
The soffit-to-fascia joint, corners, and unscreened vents — usually where something has already rotted.
How do I stop them coming back?
Replace rather than patch, use metal mesh, and trim the branches giving roof access.