Probably a good idea to leave/report or both. If he wasn't using a holster odds are 90% it was UCW.
As a side note on 5.11:
5.11 Tactical started in Modesto, California as a clothing line created by mountain climber Royal Robbins. Upon reaching the top of a climb in Yosemite California, he noticed that the trousers that he was wearing were not suited for climbing. Robbins decided that he needed to design something more durable and with better functionality. He owned a boot and clothing company and began manufacturing pants by the name of "5.11" that had a trademarked tactical strap and slash pocket design in 1968.
The name “5.11” comes from the highest rock climbing difficulty level that was listed in the Yosemite Decimal System at Yosemite National Park, which was developed by Robbins in the 1950s.[5] The difficulty level is somewhat jokingly defined as, “After thorough inspection, you conclude this move is obviously impossible; however, occasionally someone actually accomplishes it".
link
5.11 may be a "tell" but they are designed for more than just "tactical" stuff. They still work great for climbing too.