(Sorry NuttMegg - I meant to respond to
@MsBetsy!)
The drone didn't detect them in the tanks. Here is what the affidavit says:
In a subsequent interview Chris (BLACKOUT) asked to speak with his father, who was present at the police department. Chris said he would tell the truth after speaking with his dad. After being allowed to speak with his father, Chris stated after he told Shanann he wanted a separation he walked downstairs for a moment and then returned to his bedroom to speak with Shanann again. While in the bedroom, via baby monitor located on Shanann's night stand, he observed Bella "sprawled" out on her bed and blue and Shanann actively strangling Celeste. Chris said he went into a rage and ultimately strangled Shanann to death. Chris said he loaded all three bodies onto the back seat of his work truck and took them to an oil work site identified as CERVI 319 with GPS coordinates 40.21624374, -104.36667.
Chris said he buried Shanann near two oil tanks and dumped the girls inside the oils tanks.
Chris was presented an aerial photograph of the tank battery area and identified three separate locations in which he placed the bodies.
Prior to Chris' confession investigators arrived at CERVI 319 (with consent) to initiate a drone search of the area.
At approximately 4:15 PM investigators spotted a bed sheet in the field near the tank battery. The sheet matched the pattern of several pillow cases and a top sheet recovered from a kitchen trash can from the Watt's residence early that day. That searched was conducted with prior consent provided by Chris.
The drone search also revealed fresh movement of dirt consistent with a clandestine grave near the oil tanks.
https://www.courts.state.co.us/userfiles/file/Court_Probation/19th_Judicial_District/caseofinterest/2018CR2003/001/Warrantless Arrest Affidavit.pdf