In case there are several persons liable in a Notice of Disallowance, and some were already retired or separated from the service, could those who are still connected with the Agency be directed to settle the liability?

Yes, especially the approving/certifying officers of the transaction who are still with the Agency due to the solidary nature of the liability. However, the audit team and the agency could still demand settlement from persons liable who are no longer in the government service as provided in Section 35 of PD 1445.