p.191 of SR4A Astral Perception,
"Non-magical and non-living objects have only gray, lackluster
shadows rather than auras, but pick up impressions from being
in contact with living auras. Assensing can read any impressions left
behind on an object."
Nothing in the manifesting section on page 193 alters that.
"Manifesting
If a purely astral form such as a spirit or an astrally projecting magician
wishes to interact with the physical plane, she must manifest.
Manifesting is a psychic effect that allows an astral form to make
itself visible and audible on the physical plane through an act of will.
Manifesting takes a Simple Action to engage or disengage. Manifesting
characters and spirits appear on the physical plane as ghostly, hazy
images and may freely communicate with physical characters. Unlike
the Materialization power of spirits (p. 296), manifesting does not
create a physical form, and so the character cannot physically interact
with anything, nor can she be harmed by physical attacks. Because
manifestation is a psychic effect, manifested characters cannot be detected,
recorded, or affected by technological devices. Likewise, manifesting
beings are still subject to astral attacks."
I seem to recall in previous editions it was even more explicit on this point up to and including fiction materials describing trying to read on the astral plane as a frustratign experience because the text all came back as gibberish.