This I have to comment on: if you have messages you don't want a spouse to see, and you can't just tell your spouse - "hey, I got something I don't want you to see on my phone, mind not looking" - you should reconsider whether you should be married.

The former, because secrecy/dishonesty is already a major problem, and barring rather narrow corner cases, causes FAR more social problems than it solves; it is far worse for relationships. At best it means distrust and suspicion, which wear a relationship down. At the worst, it'll lead to horrible problems for some or all parties in a relationship. You should never be okay with hiding something from someone in that committed of a relationship with you - either because of love, or because of respect for them as a person.

The latter, because I realize that might be odd for some people - if you have an actually healthy, well developed trust between you and your spouse, that kind of thing does work - and it gives room for stuff like planning surprises; if you don't want your significant others to see messages of that nature, then that's another matter entirely, but it's better achieved by telling them not to look, as in a healthy relationship that in itself isn't a problem.

Now that I'm done being preachy, onto the main idea:

I think this is actually a very good app idea. Thinking about it now, I can imagine at least in part how it would work - the .desktop file executes a script, which, if it's off, runs as a daemon and flips the **** (using eSpeak and/or sound files and/or other stuff) to yell/alarm-noise when the accelerometer changes value by some easily calculated delta in all three dimensions. Then if it is executing, running it again will tell it to stop.... Off the top of my head that might not work perfectly, so a better way would be make the daemon a separate script/binary, and the first script checks if it's running, kills it if it is or launches it if it isn't. If you can intercept something like cam-key and lock slider presses, something like double half-press of camera key and slide unlock switch could also terminate the daemon, allowing for you to use the right combination of presses on the device itself to shut off the alarm before it spazzes and makes everyone look at you funny.

Why don't you just lock your phone? Simply not wanting someone to read your messages is analogous to not locking your home doors but only setting the alarm: if someone was determined to get in, they wouldn't care for the alarm going off.

press Power button > Secure Device

Yeah, for 98%, I want it work like you describe it.
But I'd like it to be very simple to use, just like Recaller .

This is not always the case.

You know imagine am taking a shower for exemple, my nephew may come and take my phone silently and throw it from the balcony or in water I let U imagine the rest. Or you imagine if he use it like a car toy with the screen as the wheel? Or use my keys to scratch my screen, you know this one already happen. I don't know why but my phone is "his" favorite toy. So all the stuff I listed may happen soon and put a code is useless in those situation, and worst I hate to enter my secret code whenever I want use my phone, no, just when I want it.

take it in the washroom while you shower?

honestly, if something like that ever happened, you have no one to blame but yourself, no app is going to help you.

why would you leave your phone lying around knowing very well that's his favorite toy to destroy? would you leave your wallet around when guests are over at your house?

don't leave your phone lying around, simple as that. i don't think it's that hard

Hello,
Well I finally found it: HandsOff.

Thank for your help.