Tracked down a repetitive signal

Sense hasn’t isolated this device and because it’s so small it probably won’t ever recognize it. But I’ve been seeing this signal for weeks and decided I could track it down.

The signal is about 10 or 12 W, repeats day and night at 4 minute 20 second intervals and has a small power dip before an increase.

I figure that signal looks like a battery charger, on a battery that’s full. The dip is to check the battery voltage, the increase might be to restore what the test used? I have 3 laptops sitting on their chargers, and three UPS’s.

First guess was the main laptop. Unplugged the charger, blip still occurred right on schedule. Second guess was the home alarm system. Went up and unplugged a lead from the battery. When the next blip was scheduled it wasn’t there. The alarm panel popped up a trouble light and began a repeated chirp. I reconnected the lead and when the next blip was scheduled it was back and the chirp stopped.

The alarm system is 25 years old, so I don’t know if the manufacturer name is important. The battery inside it is 12V 7Ah.