The skill system in Nazarick (at least in the WN and LNs) distinguishes between "Invisibility" (used by Lupu etc.) and "Perfect Unknowable", which is what Bone-daddy uses and not only erases your visual appearance but also sounds caused by the user and IIRC even their smell. The only example of Ainz using Perfect Unknowable I remember of the top of my head is (from the manga perspective) in the future so see the next spoiler at your own risk of well .... spoiling yourself
In the dwarves' caves while sneaking up on one of the major side characters of that novel
@Shyning Not that he can voice his dissagreement to the Guardians at this point and have them dissapointed in him. If Ainz is clever, he will just tell them to put it on lower priority till other matters are solved and close the case. He tried already that but clumsily, already in this chapter.
@Abetillo thats only because even if he looks and acts mature he is just mere months old, he has no actual experience on many mundane things such as dates.