Vineger Type smell inside the Car

Hi, this may seem a Joke but on occasions I can smell a vinegar smell? in my Skoda Yeti 1.2 Petrol this has been happening for twelve Months at first I thought it could have been me but my Wife has smelt it and the other day I had my Grandchildren in the Car and they suddenly said they could smell something they described it as a vinegar smell. this has convinced me it wasn't just us me and im beginning to worry where it is coming from. My Car is coming up to 2 years old its service is due in two weeks so any info would help. Cheers