I have, and even as an LEO I had to do the same thing you will.
I announced to the screener that I needed to lock up a firearm, and they had a Deputy Sheriff walk with me to a lockbox. He looked over my ID first, and watched as I put my firearm in the box.
I have no idea what he is going to want to see for ID, I had creds.
I was at the downtown courthouse rather than the RJC, but they are both King County Superior courthouses. The RJC is a nicer facility, but the downtown courthouse had black metal lockboxes. The downside that bothered me was more than one gun was in any given box, which I thought would result in theft.
I ended up being the jury foreman on a civil case, and it was a very rewarding experience.
Pretty sure commissioned law enforcement do not have to check a firearm to go to court, unless they are the defendant in a case.
RCW 9.41.300
(1) It is unlawful for any person to enter the following places when he or she knowingly possesses or knowingly has under his or her control a weapon:
(b) Those areas in any building which are used in connection with court proceedings, including courtrooms, jury rooms, judge's chambers, offices and areas used to conduct court business, waiting areas, and corridors adjacent to areas used in connection with court proceedings. The restricted areas do not include common areas of ingress and egress to the building that is used in connection with court proceedings, when it is possible to protect court areas without restricting ingress and egress to the building. The restricted areas shall be the minimum necessary to fulfill the objective of this subsection (1)(b).
(6) Subsection (1) of this section does not apply to:
(a) A person engaged in military activities sponsored by the federal or state governments, while engaged in official duties;
(b) Law enforcement personnel, except that subsection (1)(b) of this section does apply to a law enforcement officer who is present at a courthouse building as a party to an action under chapter 10.14, 10.99, or 26.50 RCW, or an action under Title 26 RCW where any party has alleged the existence of domestic violence as defined in RCW 26.50.010; or
(c) Security personnel while engaged in official duties.