Until it hits CRTP, or you have a PFD letter in hand, always be worried. Unethical CA's will retract &reinsert to ding you harder than what's technically "legal."
The CRA reporting manual instructs debt collectors to delete their reporting if their collection authority is terminated and the debt remains unpaid.
That policy is in effect in anticipation of the possible reporting of a new debt collector, and is intended to avoid the simultaneous inclusion of more than one collection on the same debt.
If the debt collector only had assigned collection authority, they may have been terminated by the OC and the rep simply was not aware of that fact, or the debt collector may be in the process of withdrawing. If the debt collector owned the debt, perhaps they sold it to another.
If the debt collector still has collection authority and deleted, only they can tell you why. It could be for any of your stated reasons.
What is the DOFD that was reported on the debt?
If the DOFD is approx 7 years or more ago, then the collection may have been excluded from your credit report by the CRAs, and not deleted by the debt collector.