One weird thing I've seen, seems to be mostly on the SLK's, is the low coolant light coming on for no apparent reason. Seems there is a vague bit of engineering involved here: The low coolant light they also use as a signal that there is a problem with the electric aux radiator fans., so keep that in mind. If you are actually low on coolant, then you at least know that there is a cause for the light to be on.
Another thing to be aware of on these cars is they wire the sensor for the washer solvent and the coolant in series. The instrument cluster can figure out which is actually low by using a voltage-coded signal. I've seen the instrument cluster get it wrong, so make sure the washer solvent is OK, and if topping off the washer solvent makes the low coolant warning disappear, you need some extra investigation done on the warning system.