Don't assume your radio equipment is at fault, this is not to say it may be, but I can tell you from professional experience that most residential alarms systems are prone to false activation, and for a wide variety of reasons.
I've been a professional fire fighter for 24 years, and have dealt with more false alarms than you could imagine. The first thing everybody needs to know is, smoke detectors do have a shelf life, most manufacturers recommend replacing them every 10 years, but in truth they go bad much sooner. Depending on the manufacturer, and model, I have seen some builder's grade units go bad within a month, and some only lasting a couple years, to others lasting 15 to 20 years before going bad.
C0 detectors are no different, the original models marketed by First Alert were the worst, giving false alarms for no apparent reason. The same applies to many of the components in burglar alarms, door sensors, window sensors that are exposed to the elements when the windows are open, control panels, and the one thing that goes bad most often is the system backup battery, and even the connection at you telephone junction box can go south on you for no apparent reason.
If your neighbor's system was old and in need of an upgrade, then my guess is their system was failing for reasons other than your use of an amateur radio amplifier. In other words....I wouldn't beat myself up over it unless it also happens with their new system, then you might have an issue there....
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