A zero-day is a terminology used to describe a threat that is brand new, that hasn't been observed in the wild and analysed yet, for definitions to be added, so nobody detects it.
It's almost certainly not the case with you. Chances are one of the three programs you use would have found it by now, if it's been going on three weeks, unless it has an unusually good method for cloaking itself.
Not saying it's impossible, but realistically, as a non-expert, that's my opinion.
