It’s not a secret, just hard for amateurs to do. No doubt states with space monitoring equipment always knew. He just did it with a camera in his backyard and his laptop.

Also, he’s Finnish.

Amateur observations of the spaceplane indicate it is flying in a highly elliptical orbit ranging between 201 and 24,133 miles in altitude (323 and 38,838 kilometers). The orbit is inclined 59.1 degrees to the equator.

This is not far off the predictions from the hobbyist tracking community before the launch in December. At that time, enthusiasts used information about the Falcon Heavy's launch trajectory and drop zones for the rocket's core booster and upper stage to estimate the orbit it would reach with the X-37B spaceplane.