Hey Nicky!
You may be able to creatively use some of the Web analytics history fields on the Contact record to help you achieve this, alongside Workflows or Lists. If I were you, I would take a list at some users (where you know their origination) and check to see if you could use the fields First page Seen, Original Source, Original Source DrillDown etc. to see if you can see any correlations as a first step. If you do, and have workflows enabled, you could easily apply a tag on the record based off that!
Let me know if these work, and if not we can continue trying something else!
All the best,
Grant