Offboarded contractor account active

Your IdP access-review flags that an admin account belonging to a contractor who offboarded 7 months ago logged in successfully 3 times this week from a VPN exit node, then opened the admin console and viewed (not yet modified) billing and customer-export pages. The account was supposed to be deprovisioned but was only disabled in one system, it stayed active in the IdP and retained its admin role. No password reset, no MFA prompt seen in logs. How do you triage whether this is the ex-contractor, a leaked credential, or an attacker, and what do you do?