Oil droplets in 600 SEL coolant expansion tank

Two weeks ago, I had the main, engine-driven fan replaced, along with the thermostat and housing and the main upper and lower radiator hoses. The car hadn't been running particularly hot (no more than usual), but the mechanic said the fan needed replacing (105,000 miles) and why not do these other things at the same time.

In checking the coolant level a couple days ago, I noticed brown oil blotches floating atop the otherwise bright green coolant.

Did the mechanic use a dirty funnel to fill the coolant, or does it sound like a more ominous problem? The car hasn't been using oil, and there is no indication of water on the dipstick.

Is this a headgasket issue? Can the gasket let oil into the coolant without letting coolant into the combustion chamber?

Thanks for any insight on this.

Brian