Modal verbs of deduction: Must, might, could, can’t

Emma: Did you hear about what happened in the office last night?
Liam: Yes! The alarm went off at 2 a.m. Someone 1️⃣ entered the building.
Emma: I’m not sure. The security guard said all the doors were locked. So no one 2️⃣ broken in.
Liam: That’s strange. Then the alarm 3️⃣ gone off by mistake.
Emma: Possibly. But the lights in Mr. Harris’s office were on. He 4️⃣ forgotten to turn them off.
Liam: I doubt it. He’s very careful. He 5️⃣ left them on.
Emma: Well, someone 6️⃣ been inside because some files are missing.
Liam: The cleaning staff 7️⃣ moved the files.
Emma: No, they finished work at 8 p.m. They 8️⃣ been there at 2 a.m.
Liam: True. Then it 9️⃣ be something serious.
Emma: Or it 1️⃣0️⃣ just be a technical problem with the alarm system.
Liam: But the window in the meeting room was open. Someone 1️⃣1️⃣ have come in that way.
Emma: That’s possible, but the window is very small. A person 1️⃣2️⃣ fit through it.
Liam: Then the manager 1️⃣3️⃣ know more than he is telling us.
Emma: He looked calm this morning. He 1️⃣4️⃣ think it’s a big problem.
Liam: Or he 1️⃣5️⃣ worried and just hiding it well.