Infrequent, stubborn small spots are typically caused by small amounts of dried oil and dust. The oil typically comes from excess shutter lubricant and after evaporation the residue is sticky. This is common to some degree with many digital brands. I have never heard of M246 shutter oil splatter.
Occasionally water drops can dry with a similar result. The thing is - water drops have to come from an external source and I presume condensation would be the most common. In the case of dried water-dust spots, regular sensor air cleaning will minimize dust and debris accumulation.
Wet cleaning always presents some risk. I am a thick-fingered dolt and once I wet cleaned a D700 sensor which resulted in smeared oil/grease swirls all over the cover glass. I was fortunate to have a local camera shop with techs who carefully clean sensors. They fixed my botched cleaning job and the sensor was not scratched.
Thereafter I limited my cleaning to regular use of a Giottos - Rocket Air Duster. I took my cameras to the shop twice a year or when I noticed a dried, spot. I suggest you avoid less expensive air blowers. The rubber in some of these hardens over time and can deposit rubber debris on the sensor. Even with the Giottos I pimped air around half a dozen times before I cleaned the sensor.
Unlike me, many people are perfectly capable of properly wet cleaning their own sensor. Sensor cover glasses are not that easy to scratch (unless you are clumsy as I am).
“To see what is in front of one’s nose needs a constant struggle.” George Orwell