If this is their first exposure (shockingly unlikely, unless they’re something of a hermit) then their immunity may have waned somewhat since then.
The second or third time I had COVID I only found out I had it because of work, I was working on a clinical trial with cystic fibrosis patients so we were getting tested daily before we went on site. This was maybe 6 weeks after my third vaccine. I tested positive but at no point did I display any symptoms whatsoever, not even a cough or a fever of any kind. Had I not been doing regular testing for work I would have had no idea I’d had it.
How often are you tested? It’s absurdly unlikely to not have had it at this point, especially working with patients every day.