I can imagine a few different scenarios, some of which involve only one person. What matters is that all scenarios boil down to two causes: either there was a problem that was 'fixed' by disconnecting the devices, or there was a deliberate reason the system was altered.
In their UL testing, the primary concern is that there be no false alarms. With that in mind, I'd pay particular attention to the disconnected section of the system: Is there any reason that there may have been false alarms? For example, could the afternoon sun have caused a higher ambient, causing the heat detectors to trip? Or, does a megger test suggest any damage to the wires themselves?
Otherwise, there are many reasons a system might be altered. Changes in tenancy, secretive behaviour by someone, preparation for arson, etc.
It's even possible that the two 'disconnected' devices were not disconnected at all. That is, someone may have tried to add the devices to the system, and to have been unsuccessful because they didn't know enough to remove that first resistor.