Are monster abilities that mimic spells detectable by the Detect Magic spell?

Question

The Sahuagin statblock includes the Shark Telepathy trait:

The sahuagin can magically command any shark within 120 feet of it, using a limited telepathy.

I would say this mimics the telepathy spell?

The description of the detect magic spell 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.

Provided you spent an action to locate the aura, which of the following would the detect magic spell sense magic from?

• only the shark that the sahuagin is controlling
• only the sahuagin
• both of them
• neither of them

Answer 1

Yes, if the ability is called out as magic. Detect magic doesn't have any wording that limits it to the effects of discrete spells — it instead detects the “presence of magic”.

Magically commanding something easily qualifies, so a sahuagin using magical telepathy to control sharks would have a detectable aura of magic. (The shark may or may not — I can see arguments both ways that hinge on where the magic is present, and I'll think that's a call for the DM on the scene to make. Personally, it would depend on whether I decided “command” meant “control” [magical] or “given orders” [not].)

Mimicking a spell isn't the point to consider though. An ability that just does the same thing as a spell, but is explicitly non-magical, wouldn't qualify. The thing to ask yourself when considering what detect magic can do is “is it magic?”