Great care needed when IDing VLF stations

A word of caution to those of us looking for very weak signals around 8-9kHz. DJ8WX was copied here overnight on 8.970022kHz without any doubt and my grabber was clearly able to see his close-down around 0700 today. The frequency and the trace timing corresponded perfectly. Likewise with G3XIZ and DK7FC/P some weeks ago.

This evening I was looking very carefully again at the Spectrum Lab screen from the last 24 hours in 424uHz BW and readjusting audio gain ranges, screen colour saturation and contrast. Out of the noise appeared another very very faint line, not at 8.970022 but at 8.96998kHz. I've attached the screen shot here (I have overlaid it with the time for clarity).

My immediate reaction was this must be Ossi OE5ODL as this is the frequency he uses. Then I checked his grabber and saw he was not operational overnight last night! So, what looked like a trace on his frequency cannot be him. It must instead be some artefact of SL or something else. We have to be very careful when seeing traces at very specific VLF frequencies: it may be an indication of a given VLF amateur station, but without some modulation or turning on/off of the carrier a doubt remains.

Sadly I don't think I did see Ossi today after all, but I shall keep looking.