McLaren currently use Mercedes engines but, with six suppliers registered for the new era, are exploring all opportunities.
They have also been linked to a return to their former suppliers Honda, who have a partnership deal with Red Bull until 2026.
Horner told reporters at the season-opening Bahrain Grand Prix that Brown had been a visitor to Red Bull Powertrains in Milton Keynes.
"I thought he was coming for lunch," he joked.
"As a power unit manufacturer for 2026 it's inevitable that discussions are going to be held regarding potential powertrain supply," he added.
"That's only natural that we would speak with potential customers."
McLaren principal Andrea Stella said the partnership with Mercedes' High Performance Powertrains (HPP) was solid but the team wanted to see what else was available.
"I think that’s natural, it shouldn’t be too much of a surprise," he said.
News of Brown's visit raised some eyebrows given apparently strained relations between the American and Horner last year when Red Bull were found to have breached a budget cap.
Without naming Red Bull directly, Brown spoke at the time of excessive spending being effectively cheating.
Horner had some strong words for him at a subsequent U.S. Grand Prix news conference, saying it was "tremendously disappointing for a fellow competitor to be accusing you of cheating".
Brown warmed up the subject again this week in a letter to McLaren fans ahead of the season.
"While it was disappointing to see breaches of the cap last year, everyone involved has learned from the process and it’s important to see the sport is moving forward in this regard," he wrote.
"I trust there won’t be a repeat of these financial breaches in 2023."
