The problem with Listeria is that it just presents itself strangely and very weakly in the average adult. Diarrhea and low grade fever, some stomach cramps. We've ALL had those symptoms in our lives but very rarely do we feel the need to go to the ER or doctor and have things cultured to see what the culprit is... Especially if a round of Pepto fixes it. PLUS it has a two week incubation period. So there's no way to know exactly what caused it.
In this case? I wouldn't worry too much. Provided the restaurant follows food safety laws then you should be ok. Listeria is relatively uncommon in the US according to food health inspectors and you're much more likely to catch it from deli meat cut with poorly sanitized slicers.
It's not worth passing up the salad, because TBH even unwashed or poorly stored veggies and lettuce can carry listeria, or stressing out over.