There are no runtime implications here, you will only notice the errors being logged in the K2 Event Log.
"25223 Process Portal OrphanRER\OrphanRER HTML Workflow not found"
[Assembly]: SourceCode.EventBus.SharePoint15.ProcessStarter, Version=126.96.36.199, Culture=neutral, PublicKeyToken=16a2c5aaaa1b130d
Use this sample script to find true orphan events. Please note the following:
If ProcSetFullName = NULL
This doesn't necessarily mean it's an invalid entry, ProcSet will only exist if there is a deployed version of the Workflow.
If DesignVersionExist = NULL
This means there is no Design Version for the Workflow.
If Both the ProcSetFullName and DesignVersionExist = NULL, it represents true orphans.
There is no impact due to this error, it can be ignored. K2 is aware of this and is working on resolving this in K2 Five 5.2.