I tested it out and moving focus is not enough to create the error. Access seems to remember where the focus was last and searches that field. If the last control to have focus is not searchable, Access will look for the next available.
It seems the code and scenario you have presented is not the whole picture. Perhaps you are using this code behind a form that does not have a control that is searchable. The only way I was able to reproduce the error is with the following ...
Code:
'Because the user clicked the Button
'focus is currently on the Button Control
Dim str As String
str = InputBox("What is the Activity Id?", "Siebel Id")
str = "*" & str & "*"
Me.MainIndex.Enabled = False
Me.Text1.Enabled = False
Me.Text4.Enabled = False
'Now we will get the error because there
'is not a field that is available for the search
DoCmd.FindRecord str, , False, , True