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I am planning an colonial marines campaign and I know my players will ask about infrared vision. In the gear list, there is no mention of an infrared scope or helm attachment although in Aliens, the marines all used it in the hive battle, not that it helped them because xenomorphs don't show up on infrared. But the smartgun description says the gun uses infrared and therefore it could be used to detect heat signatures from hidden human enemies. Would like some input on this subject.

There is an upcoming "Colonial Marines" campaign module that may see a PDF release this year, but at any rate, should come no later than Q1.

That one should cover a lot of these kinds of detail that (often due to space constraints) didn't make it into the Core Rulebook.

In the meanwhile, though ... yes, an infrared vision device — or rather, what this actually is, a thermal vision device — would reveal targets behind obstacles that are to some degree transparent to the appropriate infrared wavelengths; e.g. smoke, fog, shrubbery, some materials like certain types of plastic or glass panes, fabric screens, etc.
It wouldn't show targets behind e.g. a solid brick wall, or an armour panel.

The difference between IR and thermal is that IR vision shows reflected near-infrared (~1,000nm range); it requires an IR illuminator (floodlight).
Thermal picks up emitted IR radiation (~10µm range) from objects and presents thermal contrasts, showing hotter objects as brighter (or darker, depending on how you've set the display) than the ambient environment. IR is only relevant in darkness; thermal is quite useful also in full daylight.

An IR image looks pretty much like an ordinary floodlit scene in black and white, with visibility gradually diminishing as distance to the illumination source increases; a thermal image is a scene where most things are shades of grey, entirely without shadows, and where certain objects shine in brighter shades, pretty much all the way to infinity.
Not to be confused with image intensifiers (a.k.a. night vision goggles). This is a different technology altogether.

BTW, this shows that the Aliens are ecothermic — they do not generate heat internally as e.g. mammals do; they only hold ambient temperature, like reptiles and amphibians — thus not sticking out in thermal vision.

So, basically, a marine with a smart gun can detect hidden enemies with a heat signature, unless they are behind an obstacle that blocks the heat waves.

So, basically, a marine with a smart gun can detect hidden enemies with a heat signature, unless they are behind an obstacle that blocks the heat waves.

Exactly so!

Heat radiation, not waves, if I'm to pick a nit — but yes, exactly so!

[tmi] For anyone interested: "heat waves" generally refer to heat-generated atmospheric ripples, e.g. the convection rising above hot objects, or a jet exhaust; "heat radiation" is electromagnetic spectrum radiation that all objects above 0K emit at a temperature-dependent wavelength. Thermal vision devices detect the infrared (~300K range) portion of the latter. [/tmi]

It's also probable that thermal imaging isn't the only way a smartgun identifies targets, just one of them. It could also have motion sensing or other sophisticated visual target identification/differentiation. I mean, Facebook can pick your face out in pictures. I imagine in 160 years, the smartgun system also has some kind of image recognition software to separate possible targets from static background elements and terrain. The Marine operator being necessary only to determine if the target is friend or foe and decide to pull the trigger and/or aim.

IR or thermal might just be for added low light capability. Thermal is also good for seeing through visual masking like smoke. It's why firefighters use it. If a military is going to incorporate a "smart" weapon system, and the individual Marines have a thermal imaging monocle on their helmets, I would assume the smartgun's thermal imager can be engaged as needed, and disabled when it isn't.

Pretty much a given.

Visual, thermal, motion detector, lidar, radar, ultrasound ... the system probably incorporates a multitude of different sensors, suitable for a variety of environments, into an amalgamated and enhanced sight picture for the gunner.