How do you know this was the only person? Others might have complaint to the property management company. Besides that, not everyone thinks that its their own home who makes them sick and they will blame something else. Its possible that a specific area hasn’t been cleaned properly so only a few apartments will be affacted and not all 1800.

The only way to see what going on is by doing another Phase II ESA study. But those are expensive and you will need the approval of the owner which will be unlikely.