Something else must be affecting your results.
I just ran a few tests using FileMaker 14.01 and WIndows 7 on both drop down lists and pop up menus. If I entered find mode and right clicked the field, I could use the Operators option to put a lone = into the field. If I then clicked perform find, I found all records where this field is empty.
Things to check: any script triggers on this field that might be interfering?
Is this a field from the layout's table or a related table? (This can fail if it's from a related table.)
Yeah, it seems to work fine for data on the layout's table, but related table data through a portal is not working correctly with the operator search. Can I remedy this?
It's actually working as designed and if this field where not a pop up and you just entered find mode and typed in the = sign, you would get the same result.
= with not added text finds records with an empty field. When the field is from a related table, the empty appearance often means that there is no related record at all--and this is different from finding a record with an empty field.
The way to get this to work is to make it an Omit find request and specify an asterisk in the field. This tells FileMaker to: "find all records in the layout's table that do not have a related record with data in this field".
Thanks for the help!