"I quickly realized we lost the Bachelor. Like we don't know where he is at all, and we're yelling and we're running around, I thought I heard him, I thought I heard some dogs, so I'm sprinting down the street, and I'm yelling his name like a lunatic," Harrison recalled of the incident.
"And then I'm calling back to the United States, like, guys, what are your thoughts? I'm like, at some point, we have to call the authorities. We decided we were going to give it about five more minutes and we were going to call the cops."
After Cassie quit the show because she wasn't "in love" with the Bachelor, viewers watched Colton storm off The Bachelor set, hit a camera out of his face, throw his microphone down, jump a fence, and threaten to be done with it all.
Colton ran off into the night with Harrison and crew members calling for him and searching an area of Portugal that was "in the middle of nowhere."
At the time, Harrison didn't even expect to make an appearance on camera.
"I happened to be there, and as you saw, I was kind of in street clothes... wasn't exactly dressed to go on," Harrison told E! News.
"But when things started taking the direction they were, and I saw him throw off his mic and start walking out, I'm like, 'Let me go talk to him.' So I start walking down this driveway, about a 100 yards or so, and I thought, 'Okay, well there's the fence, so he's going to stop.'"
But Colton, using his strength and size, managed to execute that "pretty spectacular" fence jump.
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"In all seriousness, I thought there were people on the other side," Harrison said.
"I thought he was pissed at me and pissed at us and just needed his moment, but there were [crew members] there. When I opened the gate and I realized everyone was gone, and it was just darkness in the middle of Portugal... He was just a ghost, gone."
Harrison said the production team split up and eventually found Colton, and next week's two-night finale will begin right where the Monday, March 4 episode had left off.
"Inevitably things happen that you just don't foresee happening, and once again, it's just a situation we've never faced before, and he truly, at that time, was done. 100 percent done," Harrison revealed.
"And what unfolds is unbelievably raw, emotional. It's not pretty, on any of our parts -- the producers, myself, Colton. We just were like, 'Let's just let it all hang out.' I think you're going to see it, warts and all."
Colton still has two women in the running for his heart, Hannah Godwin and Tayshia Adams, heading into Part 1 of the Season 23 finale on March 11, but it's clear he already gave his heart away to Cassie.