Originally posted by Eaglerapids -Snip-
I'm finding a lot of blown birds that just didn't show up in the histogram in the field.
-Snip-
Above quote seems to be the core of your question.
There are two possible culprits, your RAW developer or your camera settings (not exposure, but tone curve, WB etc.).
The likely explanation is a little of both.
while I am not nearly technically knowledgable enough to go into details here is the reason and the steps to get an accurate RAW Histogram in Camera.
Now, I have not tried this on a pentax body, so certain steps may or may not be possible.
The RGB histogram displayed on the camera is from the JPG file embedded in the RAW file based on your in camera jpg settings. thus it is very different from the acutal RAW capture and what the sensor "sees".
2 primary things affects it. White balance and tone curve applied by the camera.
Now the Sensor itself has different sensitivities by default per colourchannel.
If you know these you will be able to find a default WB setting, generally called uniWB that reflects how the sensor sees the 3 colour channels.
This will get you a screwed up looking imagepreview WB wise, but will reveal how the sensor sees the scene and the ratio between the 3 colour channels.
Step two is to make sure that you have a linear tonecurve used in camera, to make sure that your histogram is not misleading. I have yet to own a camera that has one built in and on my nikon bodies the neutral setting does not equal a linear tone curve and my assumption is that this is a general thing across brands.
Now with the above two things handled you will get a fairly accurate histogram display of the actual RAW you have captured, but the imagepreview on camera may look more than a little odd.
Now this actually opens up a new use of colour correction filters to make sure that all 3 colour channels are exposed to the right, but that is taking things to the limit in terms of optimising your RAW capture. but none the less I have gone this route to optimise my RAW files for critical landscape work. It means more PP work, but there is something to gain.
Now with that in plase, mae sure that your RAW converter uses a linear tone curve too.
My silient prayer to the manufacturers would be to include a linear tone curve and a uniWB setting in the camera, but so far I have heard of anyone doing it.
If you want more reading and detailed how to, then do a search on DPR on Iliah and julia borg, they are probably two of the most knowledgable people on the subject I have come across. the hompage
www.libraw.org is where you will finde some rather lengthy articles by the same people on the subject (and many other subjects).