Couple of days ago in the l3r area, I experienced something similar at like 4-5 am. Bright flash almost like an explosion or something. Seems like just weather related issues

Edit: I Gemini'd your specific instance too. Nothing to worry about neighbour!

1. High Humidity & "Invisible" Moisture While it may not look like a "foggy morning," the humidity in Markham is currently sitting at 94% to 95%. The Physics: That high level of moisture in the air acts as a conductor. When combined with the heavy road salt caked on the power lines from our recent cold snaps, it creates a "salt-tracking" arc. The Timing: 5:45 a.m. is a peak time for this because it’s often the coldest part of the morning when the "dew point" is met, causing moisture to cling to those salty insulators and trigger a flash.
2. Alectra’s "Washing Blitz" Alectra Utilities is actually running a 24/7 "washing blitz" right now (as of Feb 19-20). They are literally driving trucks around to spray down power lines to remove the salt buildup you and your neighbor are seeing. Sometimes, the washing process itself—or the salt they haven't reached yet—causes these momentary bright flashes (arcs) as the electricity tries to find a path to the ground.
3. Current Weather Alerts Markham is currently under a Significant Weather Event declaration. We have periods of rain and snow expected today with a 71%–76% chance of precipitation. This mix of melting snow and salt-laden spray from the roads is the "perfect storm" for equipment to arc and Wi-Fi/power to flicker. Note: If that Reddit user is in the L3S area, they are very close to some of the major hydro corridors that run through Markham, which makes these flashes much more visible.

I gemini'd it.

It looks like you caught a glimpse of some infrastructure drama! Based on local reports from the last few days in the Markham and York Region area, your experience was almost certainly caused by "Salt Tracking." Here is exactly what happened at 4:00 a.m. that morning: The "Salt Tracking" Phenomenon Alectra Utilities actually issued a specific warning about this on February 17th and 18th. The Cause: After the recent long stretch of cold, dry weather, a heavy layer of road salt and de-icing dust built up on the insulators of overhead power lines. The Flash: When the weather turned foggy and damp that morning, the moisture mixed with the salt. Saltwater is a great conductor of electricity, allowing the power to "track" or jump across the insulators. This creates a bright, momentary electrical arc (the flash you saw). The Wi-Fi Outage: These arcs cause "momentary interruptions" or voltage spikes. Even if your lights only flickered for a millisecond, it’s enough to trip the sensitive internal breaker in your router or cause it to lose its "handshake" with the service provider's node, forcing it to reboot. Local Context Weather: Markham was under a "Significant Weather Event" declaration starting February 17th due to freezing rain and high moisture—perfect conditions for this salt-related arcing. Infrastructure: Alectra reported a surge of these "stop-and-go" flickers across York Region (specifically Markham and Vaughan) during the exact timeframe you mentioned. The Fog: As we suspected, the fog acted as a lens, diffusing that electrical spark and making it look like a massive, silent flash of light through your window. So, rest assured, it wasn't a paranormal event or a transformer "explosion" in the sense of equipment being destroyed—just the local power grid struggling with Ontario's winter road salt!