So I dug through the Suzuki online manual
for a bit and think I found some answers:
See Figure A for a nice overview of temperatures.
The heat warning light will turn on at 120°c (Fig. A)
The cooling fan will turn on at 105°c (Fig. A)
With the Suzuki recommended coolant, boiling temperature should be at around 123°c (Fig. B), so just after the warning light turns on.
The manual also has a few ideas why it might be running hotter than 105°c (Fig. C):
- I'm pulling quite a bit of weight due to luggage
- Radiator might have some gunk inside
- Thermostat could be faulty
If others are also seeing the fan turn on due to normal operation then I guess the Zuma just runs around 105°c anyway.