Kris Lake of Chown Hardware noticed it on the store's surveillance video and said he was bewildered by the sight of people crawling out of the ground.
In the video, one person shook out his shirt before putting it back on, while another rolled around before exchanging his shorts for a pair of long pants.
"My first reaction was, 'These guys are up to no good.' Your mind goes to the movies, like, 'Did they rob somebody, escaped through the sewer and knew where to map it out?'" Lake asked.
Lake checked his surveillance camera and found more video from another night showing five people emerging from the hole.
YouTube features videos of teenagers climbing in and around underground sewers across the globe, leading some to speculate that the people in Portland are doing the same.
But it's a perilous adventure to undertake. The sewers can fill with water at any point, drowning the people inside the tunnels and tubes.
"It is extremely dangerous and ridiculously stupid to do," Lake said.
A spokesperson for the Portland Bureau of Environmental Services said venturing into a storm drain tunnel is not only dangerous, but also against the city's code.