I am starting small, one mansion, easiest setting. But . . . being very careful and very thorough, noting mistakes and subsequently correcting them, testing every wall and every surface, I reach the end - only to find that my score is 90%. I'm obviously missing something (my guess is a room or section with no obvious access). I've stepped off the edge of things, to see if there is some sort of landing area; I've printed-out the full map, checking & doublechecking. Any polite suggestions?

***Spoiler alert*** If it helps the secrets can be found in these rooms (text is in white - highlight to see it):
16298, 16498, 16895, 16901, 16902, 17103, 17301, 17498, 17701, 18297

I have 100% score so it IS possible.

I think I have located the problem. I had found all of the other secrets so kindly listed in your answer; but . . . I have never been to 16902. There is no way to get there from 16901 - one just bumps up against the wall! Is there a means from above or from below? I don't think so, but I'm open to being elucidated.

The floor looks a bit damaged in the room above, 16702. Perhaps that's the way to go.

THANK YOU, mapman! I had an absolute zero chance of finding the way in. There are cracks all over the place throughout this mansion, and this is the very first time I've ever seen a "downward" prompt. Even with your generous huge hint, it still took me a few seconds to squeeze Jack into that opening. I've learned something valuable, thanks to you, and your kindness really is appreciated! All the best!

Thank you for your kind words Rayovac. Your next task is to get yourself on to the highscores list for this mansion!

See:
http://www.actionsoft.com/mm/hdscores/scores.php