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Source can vary depending on whether or not your car is a turbo.
I'm new to Volvos, but will hazard a try. If your car is a turbo, then it can be normal for the turbocharger to pass a small quantity of oil through its seals. This shows up as a fine mist of oil in the intake plumbing that may, eventually, build up enough to appear as the small amount of oil that you found. You should not find a flood of it though.
Normally aspirated (non-turbo). If it has the crankcase vent system like the turbo, then it is possible that the oil you are finding is from oil vapors and mist that has gotten past the breather, air/oil separator, flame trap system and has been sucked into the intake. It is doing what it is supposed to do. To illustrate how it works, using a turbo intake system as the example, there is a small, about 1 inch OD, rubber pipe that joins to the large air intake hose that feeds the turbo. The small rubber pipe carries the crankcase gasses and crud to the intake side of the turbo so they are sucked into the engine and burned rather than being barfed out into the atmoshpere. If the big intake pipe or hose on your car has a smaller one attaching to it, and the other end of the smaller one goes somewhere around the number 3 or 4 intake runners, then it is probably similar to the turbo crankcase breather system and that MAY be the source of your oil in the pipe.
BTW, in either case, turbo or not, the breather system needs to be cleaned out every once in a while. The crud can build up and clog it which allows the vapor pressure in the crankcase to build up high enough to push oil out of the front shaft (crank, intermediate, and cam) seals and the rear crankshaft seal, pops up the oil dipstick too. See the FAQ.
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