In the new Ahsoka novel, we know that
Ahsoka was appointed by the Republic to lead Captain Rex and a detachment of the 501st to capture Darth Maul at Mandalore. They almost succeeded when Order 66 came down from Coruscant. Rex, who removed his control chip, managed to fake his and Ahsoka's deaths, who subsequently separated and went into hiding.
Years later, Ahsoka is a Rebel operative and suggested to seek out Captain Rex, who possessed extensive knowledge of the Outer Rim which would help the Rebels find safe havens to build a hidden base of operations at.
However, the two have lost contact with each other for years, although Ahsoka managed to somehow obtain a Separatist tactical droid's head which contained Rex's location (while Rex somehow managed to find an AT-TE to live in).
All Separatist droids were deactivated by Darth Vader and/or Sidious at the end of the Clone Wars, save for a select few types which were used by the Galactic Empire to police former Separatist worlds. Tactical droids are most certainly not one of these exceptions.
So, how did a Clone Wars-era tactical droid managed to contain Rex's Imperial-era whereabouts, and how did Ahsoka managed to obtain such a droid?