More than a dozen people carrying green glow sticks and lighting candles gathered Sunday night at a makeshift tribute near Robert's mother's apartment.
The group, which had attracted 60 people to a vigil on Friday, intends to meet every night until Robert comes home.
"We just did it to show that we aren't giving up and the family shouldn't give up either," said Steve Adams, a resident of the Southwest Boise neighborhood.
Adams and his wife, Nikki, invited friends by text to the informal gatherings and encouraged them to bring a stuffed animal for Robert.
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Those who gathered said they don't talk about who may be responsible for Robert's disappearance or details of the investigation.
"Everybody has their own opinions and their own thoughts, but we don't care about that," Nikki Adams said.
Robert, 8, was reported missing at 10:11 p.m. July 24.
At a Sunday news conference, Boise police urged residents not to listen to widely circulating rumors and to know that officers will pass on information through the media. Officers received 315 leads via the helpline and are working on those that are credible.