Scenario: The PCs have Pass without Trace active when they walk through a tunnel where an Umber Hulk has been waiting behind the wall in ambush for prey, using its tremorsense.
Question: Does the Umber Hulk detect the PCs in spite of Pass without Trace because they are in contact with the floor?
Pass without Trace:
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.
A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance.