Covid antibodies may or may not be produced and if they are seem to fade quickly. This has been widely researched and documented [1]. T cell responses to SARS-CoV-2 are a better indicator of exposure [2].
If several people were hospitalized that means several dozen to hundred people in the lab actually had COVID (hospitalization rates are low, especially among people who haven’t reached retirement age yet).
Testing was done on those people and nothing was found.
This means we would be looking at a huge coverup, not false negative testing results.
[1] - https://www.nejm.org/doi/full/10.1056/NEJMc2025179
[2] - https://abcnews.go.com/Health/fda-authorizes-cell-test-game-...