You aren't manipulating mana to create a spell, you are simply calling upon something from another metaplane. Spirits leaving astral signatures would be the same as adepts leaving astral signatures by existing.
Summoning a spirit does involve manipulating mana.
The manipulation of mana, whether forming of a spell, brewing a potion, or bringing forth a being from another plane, can exhaust or injure you.
Personally, I think the simplest way to determine if a mage's actions leave an astral signature is whether they make a test involving their Magic attribute. So that would apply to spells, preparations, summoning, binding, some of the Adept activated abilities, etc.
