Marcy,I have a 50000 watt medium wave transmitter within 10 or so miles from me and 5 other MW's that used to plague my reception on my SDR for years. Their modulation products mix in the non-linearity of an SDR and wreak havoc. I used a Noolec Flamingo + filtering them out of my Loop On Ground antenna. Sometimes one of them will make it through in the afternoon and then disappear at night. But the filter does fine. This could be the reason you see this carrier.
One other reason could be a power company PLC. I have recently run into this at my location. They are allowed to send several channels of data down their power lines from substation to substation using Long Waves. Their power lines radiate the frequencies just fine. Even though they are not intended to do so. They communicate from substation to substation. I would say it is likely that is what you are seeing. If you have an SDR you can expand your display and at each channel you will see maybe 4 sidebands of tone. Each one of those channels represent a temperature, a demand or a discrete condition. My experience now is that they don't poll all channels all of the time. But it is very likely you will see all of a sudden several carries come up at one time. Not always the same channels. The power company here polls their substations a lot in the afternoon or evening. But there is nothing that indicates they are on an exact schedule.
The power company does have one more low frequency use that I don't fully understand but it does exist. The last town I lived in used that broadband low frequency service and I could not use my SDR on low frequencies because of that.