Last night, I was doing a quick check on the WiFi circuits in my area, using inSSIDer 18.104.22.1685, on the "2.4 GHz Channels" display, not on the "Time Graph" display.
Initially things looked normal, with my own one being at about -27db, my immediate neighbours being -89 db; and the one beyond being about -94 db, all with the usual slight variations.
My one is based on a Bright Box, Channels 6 & 10, with the usual +- 2 channels either side, covering Chs 4 to 12, with a Max Rate of 300 Mbps.
The immediate neighbour is on TalkTalk, Chs 7 & 11, covering Chs 5 to 13, Max Rate of 274 Mbps.
The far neighbour is a recent SKY installation, Ch 1 only, so covering Chs ? to 3, Max Rate of 144 Mbps.
Suddenly a flat-topped, narrow spike appeared on the far neighbour's trace, at about -10 db or possibly 0 db, resulting in a change of the "y" axis, thus "shrinking" the appearance of my trace and the immediate neighbours, down to the bottom of the display.
This spike was centred on Channel 1, that is there was no obvious spread across the +- 2 channels.
It lasted for about 3 seconds, returning to normal for about 30 seconds, followed by a similar spike, reverting then to normal; and I have not seen anything like it since, bearing in mind that I have probably used inSSIDer for a total of 5 minutes since.