Santa FE SM (2001-06) :: Oily Substance In Reservoir, But Not Radiator?

So a few days ago, I decided to check the anti-freeze for adequacy. It was just replaced with the timing belt / water pump job; but I'm a pilot, and I'm accustomed to checking things for myself. Lo and behold, the hydrometer didn't even register. I thought it was stuck, but then I noticed the oily film, and got scared. And then as I investigated further, I got puzzled.

The car had been driven maybe 500 miles since the timing belt / water pump job, had not overheated, and had not lost any coolant. The level in the radiator was up to the cap. Also, there was no oil at all in the radiator (at least that I could detect), and the concentration tested fine. But there was oil in the reservoir -- but it didn't look like crankcase oil. It was clear -- like water -- and felt a bit thinner than crankcase oil. Also, the outlet at the bottom of the reservoir was clogged.

There also was some sealant in the reservoir (it looked like the BarsLeak tablets), but none that I could detect in the radiator. That in itself doesn't bother me because a lot of people (including me) routinely toss a few tablets in the coolant -- Saturn used to do it from the factory -- but it does suggest at least the possibility of a previous coolant leak that someone treated with BarsLeak.

At the moment, there's no evidence of coolant loss, no water in the exhaust, no water in the oil, and no oil in the radiator. I haven't done a compression test. I did change the plugs, and there were no obvious coolant deposits on them. They were carbon-fouled, but considering that the PCV valve was clogged and the mileage on the plugs, that didn't surprise me.

So what I'm postulating is that the previous owner's son (who was the primary driver of the car) mistakenly poured some sort of engine oil additive, power steering additive, or something else intended for use elsewhere, into the reservoir; and the mechanic who did the timing belt and water pump didn't notice it (which also kind of bothers me).

The fact that the power steering pump had a bit of chatter would tend to reinforce that hypothesis. I've been doing a gradual replacement of the power steering fluid, which seems to have quieted things down. It was black as tar when I started, so I probably should completely replace it. I may do that this coming weekend. In any case, I removed, cleaned, and unclogged the coolant reservoir, replaced it, refilled it, and everything seems fine. It's kind of baffling, though.