Welcome, PlanetClaire! Hope you continue to find Eat Your Books increasingly useful and enjoyable in the new year.
As Wester says, in EYB indexing a lemon is considered to be a store-cupboard ingredient. A recipe that uses up a whole lemon or more ("juice of one lemon", e.g.), or has "lemon" in the recipe title, requires 'lemons' in the ingredient list. The threshold amount of juice for listing lemons is a quarter-cup (2 fluid ounces or 4 tablespoons). Here's the table that specifies the thresholds for listing different store-cupboard ingredients: Store Cupboard Ingredients Table.
Zest is a touch problematic, and unlike lemon juice, isn't addressed in the table. If a recipe just calls for an unspecified amount of lemon zest, or a specified amount that is clearly less than a lemon's worth, it should definitely be treated as an unmentioned, below-threshold store cupboard ingredient. In my indexing, I've usually treated "zest of one lemon" as a cue to list 'lemons' in the ingredients. Technically, though, that may not be correct: after all, if you simply zest it, you haven't used up that lemon -- the pithy, naked fruit is still sitting in the fridge, available for other recipes.... <g>
Deborah is the EYB indexing authority; maybe she'll weigh in here.
Meanwhile, very best wishes for your upcoming move to be smooth and minimally stressful.