From Shadowrun FAQ:
Do you need to maintain line of sight (or touch, with Touch range
spells) to sustain a spell? What about Permanent spells?
A spell requires a magical link (touch, sight, material link, symbolic
link, etc.) to cast. There is, however, no requirement for the
magician to maintain touch or line-of-sight while maintaining the
spell, as the link between caster and spell is already in effect. By
the same principle, you do not need to maintain touch or line of sight
when casting a Permanent spell; the spell must simply be maintained
until the spell’s effects become permanent.
One of the exceptions is when moving around an area-effect spell, cf Core rules:
If an area-effect spell is sustained, the affected area may be moved
with a Complex action, as long as it remains within line of sight.