Unfortunately, the HP p.82 Assassin's Bell has its problems.
The Assassin's Bell is clearly not a person and as a bell has no mundane senses, so it cannot be the recipient of an ArM5 p.113f Magical Senses effect. Still, HP p.82 Hearing the Tearingis such an effect. So you can either request errata from Atlas Games or ask your troupe for a resolution of this contradiction.
Making Hearing the Tearing an InVi 49 R: Touch, D: Sun, T: Room (Base 5, +1 Touch, +2 Sun, +2 Room, +3 detailed information, +4 constant effect) effect resolves this contradiction, but limits the use of the bell to within rooms. Yes, most of the time it would stand in the lab anyway - but during an expedition caused by a wizard's war it quickly becomes useless, unless your wizard applies a trick like this.