Two color job creating five plates CS3

Question

I have a strange issue with InDesign CS3 that I do not understand. I have finally boiled it down to this:
I have a Photoshop (.psd) Duotone that uses Pantone 374U and black. I use this image in InDesign. I preflight the document, and the preflight report verifies there are only two inks used - Process Black and Pantone 374U.
If I create a pdf (choose your method either export or print to ps and distill) of that document, I get a pdf that will create two plates for the printer. (Verified by preflighting the pdf)
HOWEVER, if I make an small InDesign box that is a gradient from PAPER to NONE and place this box over my Duotone (as a background for for black text, but no text inserted - yet) and create pdf using the exact same methodology, the resulting pdf will have five plates. The InDesign document still reports two inks only.
I delete this "white to transparent" box and viola! two plates in the pdf.
This was in a book that had dozens of pages and was giving problems making the pdf (by creating the extra plates). I have simplified it down to just the elements causing the issue to make troubleshooting simple. I thought the transparent box could be corrupt, but creating a new one in a new document produces the same effect.
This is totally reproducable:
Make a Duotone using any Pantone color in PS.
Place in INDD. Delete all other swatches but None, Paper, Black, Registration and the Pantone.
Make a box containing a gradient - Paper to No Fill.
Print to postscript or export pdf.
Preflight the pdf. It will make five plates.
I do not specialize in troubleshooting these type of issues, and the answer may be common knowledge to everyone but me, but I don't understand how this is happening. If the box is solid "Paper," it creates only two plates.
Does anyone have an explanation?

Answer 1

>it's only the gradient effect that I can recreate problem free in QX (and I was working in 6.5).
Sorry, I don't know QX6.5 either ;) Does it have a spot-to-nothing-gradient?
But as said: I fail to create a simple spot gradient (without transparency being involved) that will be declared as CMYK in the PDF.
"AC" is the abbreviation of "Adobe Acrobat" that I thought was customary.
Why is "abbreviation" such a long word?
>My transparencies are flattened though
That is the step that turns a simple K-object into a CMYK-object.
If you exported a PDF 1.4 your preflight would complain about transparent things but not about CMY inks.
>I guess I'm either looking either using the wrong Preflight check settings, or I'm using the wrong settings to make the PDF.
No, you are not. A colleague told me there is a preflight solution somewhere that can check if a plate actually is used but Acrobat itself only looks if the plate is declared. Which is the case with a CMYK 0 0 0 100 dropshadow.
>are printers still going to separate onto 4 plates?
That, I would say, depends on the printer (i.e. person). I would think you tell the printer that this is a 2C job. Can someone jump in here and tell what can be expected on the your side of the pond, please?