Law enforcement sources tell TMZ ... Miley's L.A.-area storage unit was broken into a few weeks ago, and someone made off with clothes, family photos and other mementos.

There's no dollar amount on the missing items, but ya gotta imagine there's some significant sentimental value.

We're told it doesn't appear the thieves were specifically targeting Miley -- other storage units in the same facility were hit as well.

Police are still investigating the heist, but they've had some success already. Our sources say some items -- for Miley and others -- have already been recovered.