Family members of a Plainfield man who's been missing for more than three weeks are worried he might have suffered a medical emergency while driving.Morgan Johnson, 27, was last seen May 18 leaving the Value Place hotel at 6295 Gateway Blvd. He lived at the hotel and worked at nearby Brightpoint Inc., police said.Fire crews on Wednesday used boats to search ponds in the area, but did not find any sign of Johnson.Plainfield police Detective Joe Aldridge said it's possible Johnson, who has a medical condition that requires him to take medication to prevent seizures, may have had a medical emergency while driving his white 1995 Pontiac Grand Am."We're asking anyone who can look down in the ditches and see if they find anyone like that on the side of the road," Aldridge said.Aldridge said there's no reason to suspect foul play in the case, but said there's also no reason to explain why Johnson would just take off.The last time Johnson was heard from was May 20, when he called his mother, friends said. Most of his belongings were found in his hotel room and his bank accounts have not shown any activity since he was last seen.Johnson is black, 5 feet 8 inches tall and weighs 155 pounds.Avery Garrett, a friend of Johnson's, posted fliers Wednesday in Broad Ripple, where Johnson sometimes went to the bars with his cousins.Garrett said the intense media coverage of missing Indiana University student Lauren Spierer is encouraging him to work harder to spread the word about Johnson's disappearance."The thing that we're concentrating on right now is just letting people know that he's out there, getting his picture out as many places as we can," Garrett said.Johnson's aunt, Janice Johnson, said one of the last text messages she received from her nephew read, "I just wanted to say hey and let you know that I love you."Johnson's friends and family have set up a Facebook page to raise awareness about his disappearance.Anyone with information about Johnson's whereabouts is asked to call Aldridge at 317-839-8700.