Brittney Griner backed out of a planned speaking engagement this week ... after she found a note in her hotel room that she took as a threat.

TMZ Sports has learned ... the WNBA superstar was slated to talk at the Women Grow Leadership Summit in National Harbor, Md. -- but bolted from the event on Monday morning after discovering a piece of duct tape near her entryway that had the words "Gay Baby Jail" written on it.

We're told Griner -- who is gay and was, of course, once imprisoned in Russia for marijuana possession -- was mortified at the message ... and immediately canceled her scheduled appearance to return home.

The Prince George's County Police Department said in a statement to us on Tuesday its officers did investigate the scene at the Gaylord National Resort & Convention Center ... and determined the matter appears to be nothing more than an unfortunate coincidence.

"Detectives have learned the phrase 'gay baby jail' is commonly used as a video game reference," the PGCPD said. "At the time the tape was located, a large convention was taking place at the Gaylord which attracts thousands of people, many with an interest in anime as well as video game enthusiasts. At this time, detectives have uncovered no link to or threat against the guest who located the tape."

Nonetheless, the agency said "detectives are working to determine who placed the piece of tape, when it was done and the intention behind it" -- and is encouraging anyone with more information to reach out.

Griner was expected to talk to cannabis enthusiasts throughout Monday ... in one of her first public speaking engagements since her return from Russian imprisonment.

Women Grow CEO Chanda Macias told Marijuana Moment she "wholeheartedly" supported Griner's decision to go back home in wake of finding the note.