Are there any spells either the exception of magic aura which are exempt from detect magic? It seems strange to me that a cantrip, the weakest level of spell in the game, can overcome far more powerful spells.
AFAIK, Detect Magic does pick up illusions. All of them.
They still need to stand and focus on them for three rounds and make the appropriate checks to know what spells are active.
You can use some spells to help your illusion within the confines of the rules:
Another idea is to use range. If your illusion is out of the range of Detect magic, your spell is "immune".
A dragon in the sky, a colosseus behind a big hole, etc.
Ultimate Intrigue got some new ways to improve illusions:
- Magic Aura, Greater: which works on creatures and objects;
- Stylized Spell (Metamagic): +10 to the Knowledge (Arcana) to identify the spell. It uses +1 level, and you can decide to mask your spell as another, so whenever detected and the real DC is not beaten, it appears as the fake spell;
- Ruse. This is a new spell descriptor. [ruse] spells appear as another spell whenever detected or identified, you need Analyze Dweomer, Greater Arcane Sight, or similar spells of the same level or higher than the ruse spell so you can see it for what it truly is. However, none of the spells with the [ruse] descriptor presented in the book are illusion spells, sadly.
The book also offers advice on how to handle Detect Magic as the GM:
The final and most important point to note is the fact that magical areas, multiple types of magic, and stronger auras can distort or conceal weaker auras. Very few GMs use this to its full potential. For instance, the NPCs might build their base on a ley line in order to mask magic auras. If all else fails, numerous countermeasures protect against a simple detect magic spell, starting with nonmagical means such as thin layers of lead and moving to magic aura, nondetection, misdirection, and more.
Take a look at greater magic aura (see page 219) for a solid countermeasure. Greater detect magic (see page 212) allows for some interesting additional pieces of information, but it’s a 2nd-level spell, so it can never be as ubiquitous as the 0-level version. Greater magic aura still foils greater detect magic.