Scott Kilday, 35, who was a director at MJM International, which owns the Ultimo women's underwear brand, told his unfair dismissal case he was horrified to find the listening device hidden within artificial flowers.
Mr Kilday was later allegedly told the bug had been planted for "business strategic reasons" at the premises in East Kilbride, Lanarkshire, the tribunal heard.
It came shortly after Mr Kilday was asked to resign from his directorship.
The hearing in Glasgow was told matters became "difficult" within the lingerie company after Miss Mone split with her husband and business partner Michael in December 2011.
Mr Kilday, who is claiming unfair dismissal, said: "It all became a bit disjointed and, basically, there came a time Michelle and Michael could not sit in the room together.
"With both being majority shareholders it was very difficult to get any decision made, which ultimately was the fall of the business."
Mr Mone eventually left MJM days before another company, MAS Holdings, bought into the business in February this year.
Mr Kilday, of Glasgow, said that about the same time Miss Mone asked him to resign as a director.
He told the tribunal: "She said MAS had asked for me to resign as director for the sale of the company to go through."
John McMillan, Mr Kilday's lawyer, asked: "What was your reaction?" He replied: "Obviously I was not particularly pleased, but I acknowledged it with Michelle and afterwards I sought legal position on my position."
He considered resigning because he felt he could not trust Miss Mone and he then set up a meeting with Eliaz Poleg, chairman of MJM, .
However, just days before this meeting, Mr Kilday claimed he then discovered a bug in his office.
Asked how he found it, Mr Kilday explained: "It was by chance more than anything else. "We had a tanning product (at the time) and just in front of the plant pot I had samples of this product.
"They were sitting there not in a particularly tidy state. This day, when I looked over, they were in a tidy state, which I found very unusual.
"I went over, had a look in the pot and saw a cable coming out, which was wrapped around the artificial flowers."
Mr MacMillan asked what his reaction was. Mr Kilday replied he was "horrified."
The lawyer also questioned Mr Kilday on how long he thought the device had been there.
Mr Kilday said the only information he discovered was that Miss Mone had been signing in late at night to the offices using her husband's "swipe in" card after Mr Mone had left the company.
Mr Kilday said he later confronted an official from MAS, Kanishke Mannakkara, about the bug, who denied any knowledge.
But, the tribunal heard it was accepted the bug had been set up for "business strategic reasons" and that it was Mr Poleg who authorised it.
Mr MacMillan asked what Mr Kilday did after the discovery and he replied: "I left."
Ms Mone and her husband recently divorced. He is now in a relationship with Samantha Bunn, a former senior employee at MJM. Mr Mone has since set up a rival firm.
It later emerged Mr Kilday now works for Mr Mone's company Pendulum Apparel.
MJM contests the claim of unfair dismissal and the hearing continues.