As in you took some tuna out of a can and mashed some mayo, from a jar, into it? It's not like you've got a whole 50 pound fish back there you're carving up to combine with homemade mayonnaise.
And while tuna salad has many wonderful attributes, including enabling you to get both your omega 3 fatty acids AND your daily doses of toxic heavy metals at the same time, one thing it does not do is keep well. So one would assume that any restaurant tuna salad is relatively fresh.
Except that their tuna melt did NOT list fresh tuna salad as an ingredient. Just tuna salad. Which means that it may very well have been the leftover stuff from yesterday that's been sitting on a windowsill for 18 hours. Yum.
American Cuisine is so wonderful.