As per symbol of death, symbol spells can have one or more of the following triggers:
- Creature looks at a symbol
- Creature reads a symbol
- Creature touches a symbol
- Creature passes over a symbol
- Creature passes through a portal bearing a symbol
Presumably, there wouldn't be any issues with an incorporeal creature such as a ghost triggering 1 or 2, as it can see normally unless specified otherwise and I don't believe there's any reason for its sight to work unusually. With regards to 3-5, however, are there any quirks that I should keep in mind? Does an incorporeal creature passing through a door still trigger #5? Can an incorporeal creature passing through a wall trigger #3?