So unless somebody handed him an all-access security device to pin on his preppy sweater-scarf...
No, David's pin is on the jacket he wears in ST III
, when he was on an official exploration mission with Saavik.
Found it in my files!:
Marcus' citizen research pin
by Therin of Andor
, on Flickr.
assorted Starfleet pins. Centre and below:
Citizen research pin worn by Dr David Marcus in Star Trek III: The Search for Spock.
From the "It's a Wrap!" auction listings:
"A metal badge appearing on researcher costumes in Star Trek III: The Search for Spock, specifically on the costume of 'Dr. David Marcus' (Merritt Butrick). The item is pin backed, molded from gold colored metal with a slight curve. Its raised features include three hashes, four dots, and an off side rectangular feature. The hashes are meant to appear on the left side. The non raised portion is painted red. One dot and one hash are painted green. The items were never explained and could indicate rank, division, diploma, discipline or anything else. The item measures approx. 2.75 X 0.75 inches."
my guess is that the writers and/or directors either didn't know about that little tidbit or decided that it just wasn't worth caring about.
The writers wouldn't be caring about pins. That's a job for Wardrobe, in consultation with the director's underlings and the costume designer.
One of the pins sold by the old catalogs as supposedly being for the back of the strap "security" pin is depicted in the separate image at the top of the combined Marcus graphics. It was worn by the character Samno in ST VI, and is depicted with his name badges.