K30 Sensor Mark (Sometimes)

Just playing about this afternoon with K30 and Sigma 150-500. Suddenly a worm like thingy has appeared but it`s not consistent. The lens was not removed between shots. In desperation I checked the sensor and it`s spotlessly clean. I took another shot of the sky at the same focal length and nothing.

Looks like a Thunder Fly in the work somewhere

It's just a small mote of dust. Nothing to worry about. Get yourself an Arctic Butterfly it uses a small amount of static to attract the dust off and you only have to hover above the sensor for it to work.
Highly recommended