I'm no Excel expert but had the same problem as you. I solved it by
declaring the workbook  as an object. For example:

Set Wb1 = GetObject("DriveLetter:\FullPath\workbookname.xls") is the
workbook I use to read data  from for use in WB2, which I keep visible.
WB2 is also set as an object, but it is the one I open  and work  with.
I don't know if it's necessary to set it as an object but doing so
lets me address it as "Wb2" throughout the routine.

What happens is that Wb1 opens as a Hidden workbook as soon as I start
the routines from Wb2. It's never visible unless I "Unhide" it, but is
available for reading.