Agreed, it looks very seedling like and as it's outdoors it will take 4 or 5 years to reach a size where it may be able to be identified.
As far as where it came from, Sarracenia seed can take years to germinate, but 15 years is very unlikely. It is, as you suggest, more likely to have come in as a seed on a more recent sundew purchase, or is it possible its dropped out of a seedpod while you have been clearing up or moving plants?
Cheers
Steve