Law enforcement sources tell us ... most of the celebs have been hit by professional burglars, who know the weak points of a home and how to penetrate them. For example, a number of the break-ins have been through second floor balcony windows, accessed by a ladder. The reason ... many homeowners don't put alarms on second floor doors and windows.

Cops also say there seems to have been a migration ... burglars are now hitting the San Fernando Valley more than the Hollywood area, where the first wave of break-ins occurred. The LAPD is investigating 23 celebrity burglary cases ... of those, 19 occurred in the Valley.

In case you're interested, the area of the Valley where many of these celebs live is pretty secluded, so it's easy to hit a home, drive to a freeway and get lost among the zillion drivers.

Now the big question ... Is this like another Bling Ring? Cops tell us the answer is no. There are various, sophisticated groups of burglars hitting posh areas of L.A. Police say there have also been an incredible number of non-celebrity hits. These burglars are looking for nice houses with nice contents ... they're not looking for famous victims.

Still ... hard to imagine a few of these hits were planned after the celeb homeowners posted their out-of-town pics on social media.