This question was inspired by a question on whether detect magic can sense feign death. I believe it would, which raises the question of how you could foil detect magic in this case. The obvious solution would be use to Nystul's Magic Aura (also known as Arcanist's Magic Aura), which is designed for concealing magic.
However, as I read the spell more closely, I noticed an issue. Magic Aura can be used on both creatures and objects. Magic Aura has two options: False Aura and Mask. Mask is used to disguise a creature's type or alignment. False Aura is used to disguise whether or not the target is magical and the school of magic of its aura. Therefore, if I want to make my creature under the influence of a spell appear to not be under the influence of a spell, I would want the False Aura option.
But there is a problem. False Aura starts by saying
You change the way the target appears to spells and magic effects, such as detect magic, that detect magical auras.
Great, that's exactly what I want to do. However, the rest of the paragraph refers to this option's effects on objects, not creatures. One could read this as meaning that the False Aura option has no effect on creatures and is meant to be used only with magic items. However, Magic Aura also says
When you cast this spell, choose one or both of the following effects.
Mask clearly applies to creatures and has no benefit for objects (since they have neither type nor alignment). Thus, if False Aura only worked on objects, they would have no reason to let you use both Mask and False Aura on a single object (save for some extremely niche cases where the target could alternate between being an object and a creature). This implies that maybe False Aura can work on creatures, and they just used the object case as an example.
But there's another complication. Even if I can use the False Aura option on a creature, it is unclear whether False Aura will conceal spells which are affecting the target or whether it only disguises magic which is intrinsic to the target (such as from any creature summoned by a conjure spell). I am interested in the former case, where the target is not intrinsically magical but is under the effect of spells.
Can I use Magic Aura to conceal or disguise magical effects on a creature?