"Just as wizards can make buildings unplottable, they can also make themselves untraceable. Voldemort would have been found long ago if it had been as simple as sending him an owl!"
-- J.K. Rowling (JKR)
A witch or wizard can use magic to make themselves untraceable, that is, impossible to be found by magic. Owls can still find the recipient of letters, but no one could follow the owl to discover where that person is (JKR).
References from the canon
Sirius Black is able to send and receive owls while on the run after escaping with Buckbeak (c.f. GF2).