Pass without trace says:
A veil of shadows and silence
radiates from you, masking you and your companions from detection. For
the duration, each creature you choose within 30 feet of you
(including you) has a +10 bonus to Dexterity (Stealth) checks and
can’t be tracked except by magical means. A creature that receives
this bonus leaves behind no tracks or other traces of its passage.
And one of the Cranium Rat's abilities says:
Telepathic Shroud. The cranium rat is immune to any effect that would
sense its emotions or read its thoughts, as well as to all divination
With both of these, does this effectively make a druid that is wild-shaped into a cranium rat untraceable (barring undeniable direct line of sight) until one or the other runs out?
Sort of, but not really
While you could not be tracked, you could still be detected by a succesful Perception check.
Also, while pass without Trace does give you a very strong Stealth bonus, you can only use Stealth while travelling if you assume a slow pace. Otherwise, you could be detected (yet not tracked).
Other ways to detect you
There are a number of non-divination-spell detection effects on monster traits, class features, and lair actions, some of which are magical in nature.