The effect is subtle (there's no taste of fish), but it's not a "hidden" ingredient: Taste the fish sauce version next to an unsauced one; the former is notably deeper, richer, and somehow smokier.
She includes her recipe for Guacamole with Fish Sauce, along with a few more suggestions for using fish sauce in everyday cooking:
• In soups to make them more savory
• In vinaigrettes, as you might use anchovies
• Mixed with olive oil and tossed with vegetables before grilling
I'm definitely inspired to keep my bottle of fish sauce close at hand, even when I'm not cooking Thai or Vietnamese recipes.
Read more & get the recipe: Other Uses for Fish Sauce at Cooking Light
Do you use fish sauce in any unexpected ways?