I read this too fast and didn't see that it was in the garage. Most garages have some slope to them, and a leaking water heater isn't that much of an issue.*The heater is in the Garage at the same level as main house floor.
If the water heater was inside the home, then what I said below may make some sense.
Many condo associations require replacement at ten years for just that reason. Trying to squeeze a year or two out of a dying heater doesn't make much economic sense. Not when it can cause damage to the units below, and to the carpets in your home.
There will be no signs to look for when it will fail. Your heater is covered with insulation that is wrapped in a metal covering. The failure will come from the tank inside of that shell.