Holdout is the IQ/A skill of concealing objects about your person, or on other people. Their cooperation isn't absolutely required, but the objects would presumably have to be small enough not to be immediately noticed if the people weren't participating. The defaults are IQ-5 or Sleight of Hand-3. The main modifiers are the size and shape of the object, on a scale that isn't closely related to the Size and Speed/Range table, and provides the Bulk statistic for ranged weapons. Because, let's face it, weapons are what people who lead exciting and dangerous lives will most often want to get past guards. Objects that move or make noise give -1 to skill, or worse, and suitable or unsuitable clothing can give large modifiers. Clothing and equipment specifically designed for concealment can give bonuses up to +4. The opponent of a Holdout attempt uses his Search skill in a quick contest.
Search is the Per/A skill of finding concealed things. The defaults are Per-5 or Criminology-5. Things that have been concealed without use of a skill are found on a successful roll; things hidden on people require a quick contest against Holdout, or in baggage, rooms or vehicles, a quick contest against Smuggling. For obvious reasons, the GM normally rolls this skill, and there are guidelines on the type and timescale of searches needed to find small things, or the ease of finding large ones. Modifiers include type of search (pat-down, skin, or complete) and for equipment such as metal detectors, explosive "sniffers" and so on. Hyperspectral Vision, Sensitive Touch, and Ultravision can boost this skill, and extra time is very helpful, if available. Any professional guard or security person should have this skill since without it, you're just there for violence. Edit: Per the uFAQ and Word of Kromm, Search is not what you use for finding secret doors. That's actually Perception-based Traps.
Smuggling is the IQ/A skill of concealing things almost anywhere that isn't on a person. Designing hidden rooms, tunnels and the like is presumably a specialised form of Architecture. Smuggling takes bonuses for the difference in SM between the object being hidden and the place where it is concealed: an SM -15 jewel in an SM -3 suitcase would be at +12, which is why good border agents major in Body Language as well as Search, since nervousness is easier to spot. You also get bonuses for specialised smuggling equipment, probably on a similar scale to Holdout equipment.
Holdout dates from 3e, when searchers used their own Holdout skill; Search and Smuggling appeared at 4e. I've had a go at making the Bulk table into a function of SM, and it doesn't work. Bulk is affected by thickness and flexibility in ways that SM doesn't capture. The penalties for striking at weapons on B400 don't really help, since they seem to assume some movement as well as SM penalties, which is entirely reasonable.
These skills are available on lots of templates, although ones that are really
good at them are much rarer; there's a common assumption that most guards are ineffective, which is certainly a fictional trope (
TvTropes).
Caravan to Ein Arris has a guard opposing Holdout with IQ; he should probably have Search.
Action points out that Smuggling is also the skill for hiding bombs and bugs, and has equipment such as the Search Endoscope, metal detectors and special holsters.
DF stresses Search, for loot, and
Fantasy has assassin and thief templates with serious amounts of points in Holdout and Search.
Gun-Fu and
Tactical Shooting have ideas on Holdout, and
High-Tech and
Low-Tech have Holdout modifiers for many kinds of equipment.
LTC2 has a rule on improving Holdout by removing a weapon's hilt, by which it seems to mean the part known to me as the guard.
Martial Arts amplifies the Holdout rules and allows calculation of Holdout penalties for melee weapons.
Mysteries applies Holdout to poisoning people.
Power-Ups volumes 2, 3, 6 and 7 all have examples for these skills.
Powers: Enhanced Senses has several abilities that boost Search, and
Tales of the Solar Patrol has
serious smuggling.
Thaumatology has magic that boosts Holdout and Smuggling, and
Ultra-Tech has amazing gear for Search and Smuggling.
I've found Search skill absolutely invaluable in police, security and espionage settings: it's one of the things that makes high Perception so useful. There's been far less need for Holdout or Smuggling.
What have you done with these skills?