Hi Guys,
We are seeing a problem on one of our Service Activity record. On the Customers field usually we have either Account or Contact selected, but on one of the record there is a
UNKNOWN participant too (look at the screenshot below)

Can anyone give insights on how this would have happened. I need to get to the root cause of this and figure out what needs to be done.
Please don't forget to mark the answer, if it helps. Piyush