The Daily Mail reports Ono was visiting the apartment to pay her respects when the Korean man happened to get into the same elevator as her.
His behavior alarmed her so much, that she notified the building's concierge, who called the police immediately.
According to police, tourist Sungno Kim, 24, had entered using a service entrance before going up to the roof via a freight elevator to take pictures, even though signs clearly state 'no trespass.'
When he took the regular elevator back down again, it happened to open on Ono's floor and she got in.
It is not clear what Kim did that caused Ono such alarm, but no doubt she would have been on high alert to potential threats given the place she was in.
Police charged Kim with trespass over the January 12 incident and he was sentenced to six hours of community service, which he has already served.
In an interview with the New York Post, the graphic design student claimed he did not intend to cause any harm, and only went to the building after reading about it in a guidebook.
“I'm a Beatles fan. I went to see where John Lennon lived,” he said.
Other residents at the building - which include Lauren Bacall and Roberta Flack - were reportedly left outraged over the security lapse.
Since Lennon’s murder, the building has become a stop on pilgrimages for Beatles fans, and most days, dozens lay flowers and pay their respects - from outside the front door.
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