Oil in cylinder number three?

I recently took my car to get an oil change, they advised that it had some leak and upon checking further I was told that I needed to replace my vacuum pump. I took it to get the vacuum pump replaced and now I am being told that somehow, they found oil inside cylinder 3 (after taking the car for a test drive with new pump). I was also told that this was very rare and needed to be checked further. I have been looking online to see what could have caused it. Help