It is possible that some remaining oil is still making its way out of nooks and crannies in the cooling system. When the engine is running, the oil pressure is much higher than the water pressure, and any leakage will be of oil into the water. Once the engine has reached operating temperature, the water system is also under pressure and this pressure will persist for some time after the engine is stopped. During this time, when there is zero oil pressure, leakage of water can take place into the oil and will eventually find its way into the sump. Small amounts of water will evaporate and vanish through the engine breather when the engine is running, Larger amounts can lead to emulsification of the oil and will be noticeable. A possible sign of water in the oil is "mayonnaise" on the filler cap. Water droplets may also be present.
In order to simulate this, you can carry out a pressure test on the water system with a readily available test kit. They are a bit pricey though, but it may be possible to hire (or borrow) one from a friendly garage or hire facility. Drop the oil out of the engine and leave it with the pressure on the engine and a container under the sump to catch any drips that may appear.Any external leakage will also be noticeable.
2008 2.0 TDCi Zetec