How to make sure smoke units are safe of engines just sitting and not in use

Smoke units or not. It's a good idea to park trains on an isolated section where the hot wire to the track is on a toggle.

All Lionel engines are still powered when not in use on a track that's live. It's merely the sound that is off. Newer legacy models are no different although the lights will be off along with the sound.

Powering down storage tracks also prevents collisions in the event a turnout inadvertently gets thrown in the wrong direction, or a parked engine accidentally gets addressed.

Yards, sidings, storage tracks etc. All should be isolated from the main line (via a break in the center rail)  and on a separate toggle for power.

Note: only the center rail needs to be isolated. The outer rails can remain connected.

I never part an unused locomotive on a powered track.  FWIW, there are some TMCC and early Legacy models that leave the smoke unit on, even when you did a proper shutdown.  I've never seen an RCMC or LCP2/3 equipped Legacy model leave the smoke on when shutdown, only the Early Legacy R4LC models.  The FEF is an example of one of those early Legacy models.  My sense is the smoke regulator is missing the command to shutdown even though the fan stops running.  That's also the sign that the regulator is likely to soon fail.

Is the regulator some thing that is easy to replace? It smokes very well at crawl speed. It’s almost impossible to see when it’s going at a good pace, though.

@steam posted:

Is the regulator some thing that is easy to replace? It smokes very well at crawl speed. It’s almost impossible to see when it’s going at a good pace, though.

There are no replacements for the smoke regulator, so if you have an early Legacy, you're out of luck.  For TMCC, you can rewire the smoke unit, replace the resistor, and run it directly from the R2LC to eliminate the regulator.