Points
1. Although the Evard's Black Tentacles effect could have been cast by a caster with greater invisibility and silent spells (who never materialized, by the way), a magical trap is still the most logical (and simple) conclusion.
2. The party never voiced any opinions about what the trap was or made any attempt to investigate its effects. They certainly did not go hunting for some invisible caster either.
3. Although I could have been more descriptive, I was under absolutely no rules obligation to reveal anything more than I did without a Search check. Even if the player made an incorrect conclusion about the nature of the trap, I had no reason to clarify it for him without some effort on his part.
4. In the future, I will provide additional information regarding triggered traps, but under no circumstance am I going to be as crass as to say: "You have triggered a magical trap" without some attempt at investigation by the PCs.