Have you checked in Patch Management > Pre/Post Procedure as this is a common place to set these warnings.
The simplest way however is to go to Agent > Agent Logs - Choose a machine that you know when the message appeared - choose agent procedure log and see what procedure ran at that time. This is the only way that your Kaseya server can be doing this. As HardKnox says whoever deleted his account will have taken ownership of the procedure.