Just a quick [heh!] update...
One, the ham who had offered, almost a year ago, to help me track this signal was none other than the late N6IDF. He seldom had any opportunity to be out around my area, and he was never able to detect it on those few occasions when he happened to be passing through, so... No help there. N6IDF, silent key, 2006.
Two, I have decided that my fixed station's antenna - an indoor-mounted "portable ARES/RACES" 2m vertical dipole - needs upgrading. [Apartment dweller. Best I can do.] Toward that end, I've started putting together a simple dipole using a BNC-to-binding-post gadget [Pomona Electronics #1452, if you want to look it up] and a couple of lengths of unbent coathanger. I don't have a decent feedline for it yet, which is why it's unfinished, but I do have about six feet of RG-58 [?] with BNC's on both ends. This means that I now have something I can take mobile that IS both portable and directional [when held horizontally], so I can probably get some more accurate bearings with this than I can with any duck, though not as accurate as with, say, a Yagi.
I seem to have been correct in my assumption that it's west of my home. I haven't gone out a-huntin' with this little dipole yet, but while experimenting, I found that I could indeed null out the signal with it. I need to try from a few more locations, although I have tried to mobile DF it a couple of times before with my HT - most recently early last September, as I recall - and had no luck even detecting it. I have since installed an IC2100 in the car. Between that and the HT-dipole combo, I might at least be able to narrow down the general area of the source. I don't expect to be able to pinpoint it.
I also mentioned this signal on QRZ recently, and was contacted by an OOC [Official Observer Coordinator], so at least now they're aware of it too. I don't know whether anyone else has tried to find it, though. At any rate, the phantom signal remains active, generally following its usual pattern of showing up sometime during the night, never identifying or carrying any kind of audio or data that I've ever heard, and shutting down sometime during the day. [That's simplifying it a bit - this just seems to be the most common pattern.] I am more convinced than ever that its center frequency is 145.2125 MHz.
It is most definitely not a receiver birdie. Birdies do not show up at the same frequency on multiple receivers of different makes and models, do not appear and disappear, do not have directional bearings, cannot be nulled out, and cannot be attenuated. Something is radiating this signal. Could be just about anything. <shrug> I still suspect an origin somewhere around San Gabriel, though. It's not causing any truly serious interference that I'm aware of, but just the fact that it's a signal that goes for hours at a time with no apparent ID or purpose makes it worthy of attention IMHO.