This warning, spotted in The Clementi Mall, makes it sound like the belongings themselves are dangerous, like a sign that says “Beware of Dog” or “Watch Out for Falling Rocks”, though admittedly neither of those warnings starts with ‘please’.
Of course the intent is something like ‘take care of’, and ‘watch out for’ sometimes has this meaning. “Watch out for your children” means ‘keep a lookout’ so that they come to no harm, but the meaning doesn’t transfer to inanimate objects as nicely.
While we’re nitpicking, we might as well point out that the exclamation mark seems extraneous. No final punctuation is needed since the words are all capitalized. Alternatively, only one capital letter is needed, since it’s a sentence with end punctuation.