Is it probably a false positive (as only one of 40 scanners found something)?
yes, but you never can be sure ...
BTW the damage done by none viral software is much underestimated so only checking for virusses might be not enough.
The real costs of damage is often "how long does it take to repair my PC" and "how much is lost" Making backups regularly (to another type of OS of course) is often equally important than using 40 scanners. I am wondering how your disks perform with al those scanners? don't they wear out?