Brooke Mueller -- Child Services Investigates ... Turns Home Upside Down

The L.A. County Dept. of Children and Family Services barged into Brooke Mueller's home the night following her OD, and turned the place upside down looking for drugs ... this according to Brooke.

Brooke is telling friends, DCFS investigators showed up hours after she returned from the hospital December 7th -- as we reported, Brooke was unresponsive after taking way too much Adderall.

Brooke says investigators tore her house apart for six hours, searching for signs of drug use, but finding nothing except a lone cigarette butt on the floor of the master bathroom.

We're told DCFS grilled Brooke ... to find out if she smoked indoors with the children present. We're told Brooke told investigators she smokes in her bathroom with windows open, far from her kids.

We're told DCFS woke up her twin boys and asked them questions like, "Has mommy ever touched you?" They also checked the boys for signs of physical abuse ... again finding nothing.

As we first reported, the kids are now with Charlie's second ex-wife, Denise Richards, while Brooke's in rehab.