I agree that it's on the sensor. The ability of bulb blowers to get stuff like this off the sensor is highly overrated in my experience. There are Lenspens made specifically for sensor cleaning, and they would move whatever this is off the sensor.
Wet cleaners (swab with solution) have also been pretty ineffective in my experience. Others swear by them, but they've never worked for me. As always, YMMV -
I think the highest probability is that it's on the sensor too.
For sensor cleaning on my µ4/3 and my previous APS-C cameras I use a #4 natural hair paint-brush in combination with a blower. I squirt rubbing alcohol on the paint brush several times and use the blower to dry it completely - this prepares a brush with absolutely no particulate matter and no oils or residues. Then I keep the blower going while I use the brush to unsettle any stuck particles. It seems to work well.
I've never cleaned 1Ds III sensors but on other canons (except 5d) a regular polyester fine art paint brush works really well. Just blow high pressure air to remove existing dust as well as charging up the brush. Then gently apply the brush. Now doe some research. On the 5d this process did not work so well. Mainly because the mirror box lubricant have been splashing on the sensor and all brushing did was to smear the lubricant on the sensor.
Better option is the loupe magnifier for sensor cleaning. I've been really happy with mine.
