Sensing Mana

Sensing Mana is a new aspect of Magery 0 introduced at the bottom of the Mana box on page 6 of GURPS Magic. It was absent in the initial version of the box on B235.
This is clarified as:
:"may sense the change when they cross a boundary between levels"
This is a Per-3 roll with a +3 bonus if "intentionally searching" although mechanically it is not explained crunch-wise what intentional searching represents.
*this difference somewhat resembles Reflexive except that is a 4-point difference
A good house rule might be to use the -3 (+0) as a "free action" version, while the -0 (+3) version could represent a mage taking continuous concentrate maneuvers.
If that is the case then a mage should probably get another +1 if using All-Out Concentrate from PP11.
*as a house rule this could be increased to +2 if introducing a Committed Concentrate for +1
Since "crossing" a boundary requires movement, a mage would probably have to use their Step while doing this, precluding maneuvers like Do Nothing or All-Out Aim which do not allow stepping.
*an exception could be made if they are riding a mount / in a vehicle as that uses it's own move to transport cargo.
There does not appear to be a magic spell to emulate this aspect of Magery, unlike other spells.
Total Spell Incompetence is specified as still retaining this ability.
==3e history==
This was present in the Principles of Magic section on page 6 of 2e's GURPS Magic. It just was not adapted to 4e's Basic Set.
Back in the previous edition, rather than being based on perception, it instead functioned on a fixed number similar to Unreliable:
*8 or less for Magery 1
*11 or less for Magery 2
*13 or less for Magery 3
The "+3 for trying to feel" did exist at this time.
Magery 0 and Magery 4+ did not exist in that edition.
It suggests using Analyze Magic "cast for the purpose" although pg 55 doesn't mention that (nor does M102 in 4e)
==See also==
*Detect Magic
*Mage Sight
*Seek Magic
category:advantages
category:rules