Ainz - Invisibility sensing skill

For that reason, he had Perfect Unknowable active and was using an illusion to disguise himself as an elf.

By the standards of this world, Perfect Unknowable was nigh impossible to penetrate, but he was using the illusion anyway just in case—an extra precaution, like he’d taken during his earlier flight. Not once had he ever believed he knew every skill or ability the world contained.

***

Ainz himself kept an invisibility-piercing ability active at all times and had to assume opponents would do the same.

For that reason, he wore a magic item—the Ghillie Guise Cloak. Anything to lower his risk of detection, but also a way to hide his true identity if he was spotted.

***

Volume 15, Chapter 2.2