The article on the [Telepathy][1] force power on Wookieepedia says, "The telepathy of most Force-sensitive individuals was not potent enough to truly read the thoughts of other beings, and could only sense the emotions or general mindset of those around them." Darth Vader could. [This article][2] says:

"―Darth Vader uses telepathy to read Luke Skywalker's thoughts and learn of the existence of his daughter, Leia Organa[3]".

However, I would think force sense would be enough to sense that someone was present and talking, without knowing what they were saying.

If he didn't just know that they were there through the force, it's always possible that he was already monitoring the thoughts of this person prior to the event. He might have considered an intrusion on his tank to be somewhat of a danger, and force sense is well documented to be able to sense that (and if he didn't, it's possible that the person was actually going to harm him, but his sensing danger thwarted this).

  [1]: https://starwars.fandom.com/wiki/Telepathy/Legends
  [2]: https://starwars.fandom.com/wiki/Telepathy