Do users have a globally unique identifier (GUID)?

I currently have offers across three different fiat currencies open. Would anyone be able to determine that those three come from the same user?

It’s not possible in the UI as far as I can tell, but what about someone sniffing and analyzing data traffic or running a modified client?

The network ID is the unique ID. You can see the onion address when you mouse over the user icon on the right side of the offers. Though the onion address is not linked to your real life identity as long as you are not trading vai bank transfer, and then only the trading peer knows your other offers.
If you want to separate all your offers it would require that you run different instances of Bitsquare.
With the program argument you can pass any arbitrary name and then you have a different data directory named after that (default “Bitsquare”).
If you dont have offer, trades or disputes open you also could renew/reset your onion address. I will add that feature in some later version, but you really need to take care, as with a new onion address you could not be reached anymore if you still had offers/trades open.