If the password manager is known to support " thousands of decoys by design" not only will they know about it, it will be very suspicious.
Of couse it will. Because morons like you prevented it from being widespread and included in every distribution as standard part of encryption. Catch 22...
Or they can demand you open the encrypted volume, and to open the hidden encrypted volume as well (that they know the software you use supports) or else. In the second case you are screwed, no matter how "plausible" your deniability is.
And if volume supports thousands of filesystems inside one volume and you turned out your primary password (with browser to browse banks and banking information, filesystem #4011) and your second password (let's say with granny porn, filesystem #517), will they demand password from filesystem #2301? Filesystem #511? Huh? At some point it becomes ridiculous. Yes, you had something that you really did not want to reveal, but here it is, officer. It's quite different from truecrypt's double filesystem with only ONE secondary volume.
Oh, and incidentally, unless you were very careful and created the hidden volume shortly before the laptop was stolen, the presence of hidden volume can be detected with enough reliability to give probable cause.
It is encrypted filesystem's task to carefully shuffle all sectors, overwriting non-used sectors with random data as well to avoid wear leveling analysis. If you boot into decoy system sometimes and browse a little (let's say doing your regular banking) then it is NOT a decoy after all, no matter how you investigate modification time.
Such a system is much more difficult to design than two-part system like truecrypt's. Doable? Yes. For example, today ram is cheap, so just load all encrypted filesystems into ram and work there, then dump back, overwriting remaining free space with random junk. Fragile, but quite doable and close to impossible to analyze. Or may be something else, but with expert morons throwing shit and derailing civilized discussion there is no such system, alas.