Can basic camcorders be used to find these infrared devices?
If these infrared units are actually in use and constantly emitting then yes (this is the reason such devices are not used.) A lot of cameras can detect the infrared spectrum, especially black and white security cameras. This is why you can see your remote control's ir signal through a black and white camera, or detect security cameras that have ir emitters.
The military and drones do use ir for tagging targets (in some situations,) and this can be detected by some cameras however a target will only be ir tagged for a very short time (like to guide a missile, or to detect a dynamic optical tag and track the vehicle by another method.)
However it is extremely unlikely that these devices, the bulky 9 volt ir emitter you linked to, are used by the military. The story you quoted about the 19 year old planting these devices, is more than likely just propaganda that aims to show that the US doesn't put much thought or care into what and who their drones destroy.
The other part of the article, where "Mr. Rehman explained that the camera could somehow detect otherwise invisible signals from the ... electronic tracking devices that ... guide American missiles," is possibly true (like in that extremely brief time frame I mentioned above,) but it is most likely either just thought to be true, perhaps from looking at recordings of targets being destroyed and/or it is method to make other terrorist's feel like they have some control or protection from being killed by a drone.
The military has many different types of tagging tracking and locating (TTL) devices. From actual cellphones to radar responsive devices, infrared tracking and targeting devices all of which have been used to help defeat terrorist networks.
Drones have multi-spectral targeting and tracking systems that include electro-optical, infrared, laser designation, laser illumination, synthetic aperture radar (SAR), ground moving target indicator (GMTI) and Electro-optical (EO)/Infrared (IR) Full Motion Video cameras.
So drones are able to detect an infra-red beacon like the one you mentioned. And as mentioned above a camera that could detect ir would also see the beacon, however it is very unlikely that the 9 volt device would be used, because it is very detectable and presumably always on, limited battery life (military trackers usually aren't self powered or are at least triggered to turn on,) and more importantly it has the possibility of causing a large amount of false positives. It is just a cheap infrared light, it can be cheaply and easily reproduced by other parties, or its light could be reflected making pinpointing harder, etc.
Optical device that are known to be used in this type of a situation are dynamic optical tags.
Known as "DOTS," Dynamic Optical Tags, DARPA claims that the system is
comprised of a series of "small active retroreflecting optical tags
for 2-way data exchange." The tags are small, 25x25x5 mm with a range
10km ... possessing a "low probability of detection," the devices can
be covertly planted around alleged terrorist safe houses which can
then be targeted at will by drones.
DOTS, like lots of other tracking devices are passive. Most of the time DOTS will be passive (not emitting light or even using power, like an RFID chip) and they only become active when interrogated by a laser with the correct code. Once correctly interrogated, the tags will begin to modulate and retro-reflect the incoming beam for a brief period of time.
For further information on tracking technology, read the Summary of the Sensing and Positioning Technology Workshop of the Committee on Nanotechnology for the Intelligence Community: Interim Report.
The military has had a long time and spent a lot of money developing reliable systems, however from time to time there are huge oversights. Like when insurgents have used programs like SkyGrabber — a program that allows for satellite data capture — to obtain access into the drone’s video feed.