Chris S.
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I clean sensors under a stereomicroscope, and can't imagine how anyone gets a sensor really clean with just a loupe. Quite a bit of dust that is easily observed under a stereomicroscope is difficult or impossible to see under a loupe. Lest someone suggest that such small particles won't affect actual shooting, this depends on what kind of shooting you do. If you attach microscope objectives to your cameras, as I often do, very small particles on the sensor can be a real problem. (And if you stack hundreds of images for extended depth of field, as I do, you want to minimize the need to digitally retouch dust spots.)
Another good thing about working under a low-powered stereomicroscope is that it's often possible to visually recognize what a stubborn spot is comprised of, which helps in deciding how to clean it off. Most of what I see, on my sensors and those of friends and family, is skin flakes and textile fibers. And if you ever examine random house dust under a compound microscope, these two components will typically form a large portion of what you'll find. Some skin flakes can stick as if glued to a sensor, and when finally removed, will leave a trail of oil behind. Given rather limited information, I'd suspect that the OP's speck may well be a skin flake.
Removing stubborn skin flakes is something I would do fairly late in the cleaning process, after several rounds of air blowing and visual examination; then one or more broad wet cleanings with alcohol (either vodka, drugstore rubbing alcohol, or hardware store methyl alcohol--not Eclipse or other expensive frippery) and inexpensive generic sensor-wide swabs (often recovered with sliced Pec Pads). When all is clear except a few stubborn spots, it's time to closely examine and remove those spots. A tightly stuck skin flake requires a small swab--at the largest, a bit of Pec Pad fastened around a pencil eraser. On the smaller end, a bit of Pec Pad fastened around the end of a toothpick. Moisten the pad slightly with alcohol, and remove the flake. Sometimes this requires a surprising amount of pressure, but I've yet to scratch a low-pass filter. Must admit that I'm tempted by Slug69's suggestion of acetone for this specific purpose. I may have to experiment. A key point here is that one would be using a very small amount of the solvent, and that it would be applied onto a very small swab. If the acetone dissolves this particular oil more readily than alcohol does, this might reduce the amount of pressure that needs to be applied to the swab, and further reduce risk.
Typically, once a really stubborn skin flake budges, it leaves an oily skid mark, which should be removed with yet another small swab. Cut portions of Pec Pads will drop textile fibers, but these do not adhere and are readily blown off.
A thorough cleaning like this might take 6-12 swabs, small and large. So reusing swab sticks and recovering them with Pec Pads is an important part of keeping the process inexpensive. Also, it's quite possible to get several useful passes out of a single swab--if between passes, you pour alcohol over the swab tip, wipe it and squeeze it with clean Pec Pads, and blow it dry with canned air.
I would not personally send a sensor out for "professional" cleaning. The sensors that friends have brought me, fresh from a small variety of professional shops, have never so far been truly clean. I don't believe any of these had been cleaned under a stereo microscope, or by someone who was really thorough. That a shop would leave a large speck near the edge of the sensor surprises me not at all.
Cheers,
--Chris
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