Thanks for all the information. I haven't got any recessed cans, and cutting a hole in ceiling would result in a half ton of loose cellulose/fiberglass insulation raining down. I don't want any of that nasty stuff coming into the high quality air of my house. Ultimately plan to make new access point in garage, but there are problems there too.
WRT other detectors, yes, I have opening detectors on all windows and doors, and glassbreak detectors on all windows. (I was reading this list way back when I had system installed, or I would have never even known to ask.) Also have deadbolts and window pins and all windows are visible from streets and neighbors. In ten years, I had one attempted break-in (or perhaps it was vandalism) that broke two windows. On one window, the detector failed to go off. Possibly it had boxes stacked too close to the detector, which is mounted on the opposite wall. Or maybe it wasn't that one that failed actually.
Then I discovered this little catch. Suppose I'm on vacation (I was on vacation at the time) and perp breaks window then runs off. Then, maybe a day later, they come back, and gain access through already broken window. Now there is no more defense without a motion.
Worrying about this possibility, I almost cut my vacation in half, at great loss and expense. However, thanks to good reporting by my neighbor, I figured out that the only important window they had broken was in the living room, which happens to be the room where my motion is. So I was safe, at least that time. But since that happened, two years ago, I have been yearning to have motions added to every room where I have valuable stuff for two reasons: Glassbreaks don't always work, and once glass is broken and alarm resets you're wide open.
Charles Peterson