Unless there is some sort of software corruption...if you were to re-boot the PC to restart 3CX and then power up each set in question. you should see a log showing the sets registering AND, then ,any activity that would indicate why 3CX thinks that the set is on an active call.
If a number of sets are showing that they are active when they shouldn't be then just concentrate on one of them, just leave the others powered down. What you want to discover is if there is an action being taken by the set itself, or if it is just happening with no log whatsoever. If that is the case then my first suspicion (and I may be wrong) would be some sort of corruption, because that isn't normal behavior.