The Sahuagin have an ability
Shark Telepathy: The sahuagin can magically command any shark within 120 feet of it, using a limited telepathy
I would say this mimics the telepathy spell?
Detect Magic says
For the duration, you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any.
Would detect magic sense magic off (provided you spent an action to locate the aura) : the shark the sahuagin is controlling, the sahuagin only, both, or neither?