Those scanners (with the little things that spun around inside) actually were recognizable as salt shakers. The ones that weren’t were used as McCoy’s surgical tools.
In The Making of Star Trek, there’s a photo of them laid out on a tray or cloth (or something). They were very, very modernistic, probably imported from Sweden or someplace like that.
With respect to Sick Bay itself, the screen for the Tantalus Field that Evil Kirk used to get rid of his enemies in “Mirror, Mirror” can be seen on the wall in many later episodes (I first noticed it in “Journey to Babel”).
Parts of the NOMAD space probe were recycled in the Romulan cloaking device of “The Enterprise Incident” and Flint’s robot in “Requiem for Metheuselah.” Things like this happened all time; you just have to watch for them.
The impressive-looking conduits and other fixtures in the corridors of the Enterprise were usually styrofoam packing forms salvaged from Desilu dumpsters and spray-painted in bright primary colors. They could easily have been broken if anyone messed with them.
This was the reason for the “green or gold” controversy over the Command level uniform shirts on ST: TOS. When viewed live or in publicity stills, they were distinctly green, especially when they switched from velour to another fabric in the third season. Because of the lighting and other factors, they usually appeared gold (or “tenne”) when seen on a TV screen.
There’s at least one company that manufactures these shirts in both colors to satisfy Trekkies with different tastes.
The various Law & Order franchises all use a single outer office with cubicles, one inner office, one brownstone (shot from numerous angles) and one bodega. The “Yankee Rebels” biker bar was reused a couple times. Several characters have worn Jack McCoy’s rep tie. One comedienne noted that all the dead sex crime victims wore matching bra and panties, “so I’m safe!”
Shooting on location and stock footage is an option too.
My brother is a CGI tech, and he said once his job was to take outdoor footage of the real location where a show was set (I don’t know whether it was stock footage or shot by a crew that went to the location), and mix it with set shots of stuff specific to thew show.
I don’t want to say what show he worked on, but if, for example, ER wanted to show it’s actors on real Chicago public transport, they would send a crew to film the platform in Chicago (or contract with a crew already there), and film the outside of the car, possibly with a double boarding it, and not pay their expensive actor to travel to Chicago. Then they would mix a green screen shot to get the on the platform, use either a green screen or double to show him boarding, and then a set, or a location shot on local LA public transport to show him riding.
Same kind of thing to make it look like CSI actors are really in Las Vegas, and so forth.
So I imagine it wouldn’t be hard to find a cooperative office that (for a consideration) would let a crew film in the office, and then green screen the actors in.