residue after HF vapor etch?

Hi all,
Has anyone seen an obscure residue develop on a wafer after doing an HF vapor etch on oxide film?  I've seen this in the past and am not sure what is causing it.  If you have any ideas I would love to hear them.

Here are my ideas:
1) condensation of water from the air causing variations in the etch.  I fabricated a teflon holder to clamp small pieces to the hotplate to ensure no condensation forms and still see the residue, so probably not that.
2) residue of silicon dust from scribing.  The problem seems to be more common for small pieces that have been cleaved.  Maybe a thin residue of silicon dust from the cleaving is coated on the surface causing problems with the etch.  I will try re-cleaning the small pieces after cleaving to see if this helps.
3) Thin layer of contamination from an earlier step in the process.  I'm reaching here. I follow all the clean protocols, but due use cmp for several layers so could be related to that.  Though I would expect it to be consistent, not intermitent, and not preferential to small pieces.

Any other thoughts or experience?

Thanks,
Chris