Hello! This morning I woke up to one of the smoke alarms installed in my house going off. I installed two First Alert SCO500s, one upstairs, and one downstairs. They’re both wirelessly interconnected, I test them every week, they both go off every week. But this morning, the downstairs one went off, but not the upstairs one. The downstairs one went off for around 10 seconds before turning off itself, but the weird thing was is the upstairs one was latching, indicating that the upstairs one had initiated the alarm, but only the downstairs one went off. My dad was downstairs when it went off, and he said that the downstairs one’s green light was blinking rapidly, indicating its trying to communicate with the other alarm while the alarm already WAS going off. Normally the green light blinks rapidly, then stops, and THEN when the alarm goes off, the red light syncs with the horn doing code 3. My dad thought that maybe some dust got in the chamber of the upstairs one, and it was in an alarm state for long enough to set off the downstairs alarm, but not itself, but that would be weird because it never takes 10 seconds for the other alarm to repeat what the first one is doing. It did code 3, but there was no smoke (probably dust or a spider or smth), but it didn’t go off long enough to do the voice. I’m just very confused as to how the upstairs unit initiated an alarm, and set off the one it was interconnected with, but not itself. If anyone more familiar with First Alert smoke alarms could tell me what possibly happened, any info would be greatly appreciated. Thanks! 
I’m not sure about the whole “one unit alarming another even though the former didn’t actually sound”, but blowing them out with an air compressor or compressed air duster should hopefully keep either from falsing (this is a necessary bit of maintenance with photoelectric or dual-sensor smoke detectors as I’ve learned with my house’s units, probably because the photoelectric sensor is a lot more sensitive to dust than the ionization one (though you should clean any detector regularly regardless of its type just to be safe anyway).
The green LED flashing rapidly might only be when the detector is being tested: in an actual alarm it might go straight to sounding the horn & flashing the red LED instead (which makes sense since when smoke or CO is present every second counts!).
Perhaps something else you could try doing is testing both units one at a time & having both you & someone else observe what each detector’s LEDs & horn do, which might give you a clue on what happened.
I will definitely use my air duster to clean the sensor, as that is part of my maintenance routine. As for the LED thing, in my experience, every time I’ve tested those two alarms with smoke, they do the same green LED communication blink for about 5-10 seconds before going off, same as testing. And they definitely weren’t testing, as the alarm didn’t say “testing”, and because they’re SCO500s, the test cycle is “testing”, 2 rounds of code 3, “warning, evacuate! smoke in (location), evacuate!”, 2 rounds of code 4, “warning evacuate! carbon monoxide in (location), evacuate!”, “Highest carbon monoxide level was (0-999) PPM”, but this morning it just did 4-6 rounds of code 3, nothing else. Thanks so much for you response!
Ah yes, Le Manuels… As far as I know any wireless interconnect First Alert, including the infamous “PLEASE SEE MANUEL” alarm, is known to do that.
I think when the green light rapidly flashes on the initiating devicical after not detecting any more smoke or carbon monoxide, it is signalling the other devicicals to stop going off. Although that’s my theory.