as far as security goes:
riot shotguns are loaded with tranq darts by default. contrary to what the name implies, they don't stun, and are loaded with haloperidol, making them effective against stimulants. an interesting idea, but far too situational to ever see use.
stun batons have removable batteries, which can be handled with a screwdriver. this enables you to potentially reload a baton in the field or replace the battery with a higher-tier one from science.
seclights are surprisingly robust, being only marginally less harmful than a stunbaton. likewise, tasers are also surprisingly potent melee weapons. generally, both of these are overshadowed by the baton. on the topic of batons, a baton can be used on a shield to make an intimidating noise.
handcuffs can be used on orange (and only orange) shoes to make leg-cuffs. these force a person to remain in walk intent.
trying to wash a live stunbaton makes you electrocute yourself.
the DNA and fingerprint strings in security/medical records can actually be altered, potentially allowing for elaborate frame jobs or erasure of evidence.
the captain is, similairly to the chaplain, innately immune to being converted by cultists, even if their implant is removed. i have no idea of the reasoning behind this.
during rev, both heads and revheads are considered dead if off z-level. this can allow for potentially non-lethal resolutions to the gamemode.
targeting the mouth and clicking someone in help intent with a ranged weapon will perform an 'execution'. This takes a long time to prepare, but deals 5x the damage of the weapon to the target. most weapons do not deal enough damage for this to actually be lethal to a healthy victim.
a few not-useless-but-fairly-obscure mechanics:
tracking implants can be used to teleport directly onto the target as one would a beacon. this makes re-capture quite easy.
riot armour has a very long strip delay and protects almost all of your inventory slots. combined with a flare in your pocket, this confers immunity to shadowling conversion.
chemical implants can be activated by the person implanted in them, not only remotely. this allows the construction of ghetto adrenal implants.
flashbangs, being grenades, can be combined with the soon-to-be-removed explosive lances. this makes a ranged stun weapon that is effective even against people otherwise immune to them.
roundstart members of security heal when consuming donuts. donuts are also not considered junk food, meaning they're a fine substitute for healthy eating (in ss13).
clicking a magazine-based weapon with a magazine will perform a tactical reload, swapping the existing mag for the new one in a single click. this ditches the old mag. weapons with multi-stage reloads such as the SAW cannot do this.
the combat knife can be sharpened with a whetstone as other knives, making it arguably the best non-stun melee weapon to the crew.
prison jumpsuits cannot have their suit sensors disabled.
(not too obscure anymore) tape rolls can be used to completely silence footwear. this is most notable for disabling the distinctive noise of jackboots.