[Brian]

The item got checked off, in other words - it didn't really 'go' anywhere; it sounds like the option selected under View -> Status Filter is set to 'Remaining', 'Available', or 'Next Action'. If you select 'Any' or 'Completed', the item would reappear.

As to how it happened, it's likely that the checkbox was accidentally clicked, or that you hit the space bar while in row selection mode. (When a row is selected, but doesn't have a text edit cursor inside it.)