By RAW, no, but ruling that way causes problems
RAW says very little about what is or is not detectable, but it does clearly establish some things as detectable, and Momentary Stasis is not one of them.
However, at my table, most actions that target other creatures are detectable by default, unless the rules clearly intend it to be otherwise. I believe that this ruling removes more problematic edge cases than it creates.
RAW does not prevent Momentary Stasis from being the perfect crime
Some class features specify some kind of motion in their ruleblock. For example, Lay on Hands specifies "As an action, you can touch a creature and..." - or Bardic Inspiration, which strongly implies that it makes noise but doesn't outright say what that noise necessarily is, "You can inspire others through stirring words or music. To do so, you use a bonus action on your turn to choose one creature... who can hear you."
Others, like Channel Divinity and most Warlock and Wizard class features, including Momentary Stasis do not specify any motion or sound.
One could make an argument because they lack any wording to the contrary, activating these features is entirely mental and has no detectable presence.
The problem with this ruling is that it is a non-obvious reading of the rules. It grants certain abilities additional utility based on words that are missing from their rules, rather than by the words that are present.
This is ambiguous RAI, ambiguously playtested, and leaves room for careless designer error. There's no way to know if the designers intended for those abilities to be usable that way, or if they simply forgot to put anything there and then didn't catch it in playtesting.
This causes a couple problems
- It is completely one-sided. If something is undetectable, then it can be done unerringly even against other PCs, and the victim can't respond at all without metagaming. While bullying dynamics between players like this are best solved outside the game, I think the game works better without one-sided interactions even if they're only pointed at NPCs.
- As an example of this, I was playing once in a group once with a sorcerer who would Subtle Spell Counterspell his own teammates when things didn't go his way. I don't hate intra-party conflict, but having it only occur in situations where I couldn't respond at all sucked a lot of fun out of the game. (Subtle Spell itself is clearly intended to be stealthy and I would exempt it from detectability-by-default, but I would advocate against adding more one-sided interactions to the game than are necessary)
- It crowds out abilities that were clearly intended to be stealthy. With this ruling, a School of Conjuration wizard is less likely to get caught creating a distraction than a School of Illusion wizard, because Minor Conjuration doesn't specify any signature while Minor Illusion has somatic and material components.
I argue that RAW is ambiguous, not decisive
There is no rule that says this action is detectable.
However, there is also no rule that says actions are not detectable by default. For physical actions (like attacks) we can fall back on our knowledge of the real world to determine if something should or should not be detectable. For magic actions, we have no common sense to fall back on. I believe that the game works better if one rules that magic actions have some detectable signature which needs to be deliberately disguised or hidden in order to use them secretly.
At my table, players tell me what their magic looks like
If the text of the action doesn't describe what it is, the default ruling at my table is that all actions are physical actions - but that if it's unspecified, then the players get to describe what the action looks like.
I make allowances for rule-of-cool or players who want to be sneaky; incantations can be whispered, gestures can be done under the table, an unblinking stare can be disguised in a crowd of people, but it's never free.
If undetectability is the default, monsters can make much better of it use than players
If your players are unconvinced about the fairness of this ruling and think you should rule the other way, stress that the ruling apply equally to monster abilities. Many monsters also have actions in their stat blocks which are neither attacks nor spells, and which don't have defined signatures. These include things like the Breath Attacks of Mephits, many of which also have the False Appearance trait.
I think most players will realize how undetectability-by-default is unfair as soon as you even suggest the possibility of pointing the same rules back at them.
Addendum: The Invisibility Spell
Invisibility, by RAW, ends only if the caster attacks, casts a spell, or stops concentrating on it. I extend this to also include any class and monster ability which requires an action and affects a hostile creature.
This is a house-rule, not a ruling, because it explicitly goes against RAW, but I believe it makes the game better. As with undetectability-as-default, I believe this house rule this removes more problematic edge cases than it adds.