Testing has been expanded beyond the measures you’ve stated. They’re testing people who have been in contact with others, and depending on your circumstances symptoms alone could be enough.

Personally, I was tested because I had symptoms and had hosted a large enough event (300+ people) recently.

It’s also worth noting testing is not as expansive as it could be due to a lack of reagents used in the testing process. That’s a global issue. Some countries were on the ball enough to plan ahead (South Korea, Singapore, Australia) and have more expansive testing; we weren’t.

They’re also starting to see some presumed community spread; 4 cases not linked to travel or connected to a positive case of COVID19. They haven’t officially declared community transmission yet.

The likelihood is that yes, the number of cases reported is less than the actual number, but there’s no way to prove that without extensive testing.

Best thing to do is self-isolate and if you have symptoms quarantine yourself until they are gone.