Tremorsense: A creature with tremorsense is sensitive to vibrations in the ground and can automatically pinpoint the location of anything that is in contact with the ground. Aquatic creatures with tremorsense can also sense the location of creatures moving through water. The ability’s range is specified in the creature’s descriptive text.
Supposing vibrations in the ground work at all like they do in our world (a necessary base assumption to make sense of the fictional world at all), they are reflected at interfaces where the speed of sound changes sharply. Hence, if there are any vibrations around, the tremorsensing creature can detect at least the close boundaries where the density of the material changes sharply (for example, from rock to air or water or I guess magma or treasure chest), and probably also where the kind of rock changes.
The relevant field of study is inverse problems; especially inverse scattering and inverse problems for the Helmholtz equation. (Vibrations are sound/pressure waves.)
It seems that an earth gliding creature creates no vibrations (see quote below). Exactly how sensitive the creature is, and how much background vibrations there are in the medium, are completely up to the GM/referee.
From earth glide monster ability:
Its burrowing leaves behind no tunnel or hole, nor does it create any ripple or other sign of its presence.