Despite massive search, no sign of missing 7-year-old Willow Long

(CNN) -- Police don't suspect foul play in the disappearance of 7-year-old Willow Long from a rural Illinois home over the weekend. But they're wondering if the movie she'd been watching at the time may have something to do with it: Disney's "The Princess and the Frog."

"We were thinking maybe she wanted to go on this adventure," said J.R. Nieman of the Watson fire department. "So we have checked bodies of water."

Willow was watching television with her brothers on Sunday when her mother, who wasn't feeling well, laid down for a nap. When she woke up, her daughter had vanished.

When she asked her 3-year-old son where her daughter was, he pointed to the front door.

Willow disappeared sometime between 9 a.m. and 10:15 a.m. The second grader left her glasses at home.

Willow evidently changed clothes before leaving her house. Police said the clothes her mother described her wearing when she last saw her were found in the family home.

Officials have covered about 18 square miles as of Monday evening, telling reporters in a news conference, "We want to make sure we cover everything."

There are several agencies helping with the search including the FBI, Effingham County Sheriff's Office, the Watson Police and the Illinois State Patrol. No Amber Alert has been issued for Willow, since foul play is not suspected.

Search dogs initially picked up trails around the family property, but there have been no other leads.

More than 300 volunteers have been helping to "double and triple search" as much of the area as possible, which is a mostly rural landscape.

The heat wave across the region has not helped; multiple volunteers have been taken to the hospital with symptoms relating to heat exposure, authorities said.

The volume of volunteers, while inspiring, has caused some confusion.

"There were some city folks that got lost in a corn field," one volunteer told CNN affiliate WICS. "They kept turning when the corn rows turned instead of walking straight through the field."

The heat has also caused complications with the thermal imaging technology that officials are using.

"We've had several planes and helicopters trying to find Willow," Effingham County Sheriff John Monnet said Monday. "But the fields are so hot that it's become harder to pinpoint the glow of body heat."

But investigators remain determined; "We have one goal; that is to find Willow".

Willow Long has shoulder-length red hair, is about 4 feet tall and weighs about 50 pounds.