The Egg

The script for ALIEN suggests that when lying dormant, the egg is opaque with an organic and bumpy surface, but when "awakened", it gets transparent so that by shining a flashlight or similar on it, you can almost see through it, revealing its deadly fruit. This could be a way for the facehugger inside to see it's prey outside, but that is as of now still unknown. Apparently the egg seems to be the only form in the Alien lifecycle that doesn't have molecular acid for blood, but I could be mistaken.

Surrounding the base of the egg are several small "tubes" and "tentacles". The function of these are as of yet unknown, but it's speculated that they help the egg sense movement outside so that it knows when to react. If this is true, it is also speculated that the egg must be able to sense how big the intruder is, if it's a member of the Alien hive and if it is worth a try to open up.

[The Following Paragraph was written and added by Mike Lynch, author of the Anchorpoint Essays and Attention ALIENS & PREDATOR Collectors]

As for how the facehugger/egg locates prey cocooned into a wall: one theory may be that the drone responsible for the actual cocooning excretes a pheromone (A pheromone is a chemical "signature" that is excreted and used by many insects and even some animals to signal territory, convey messages, or even as a means of identification.  Ants from one hive will attack intruding ants of another hive because the pheromone signature doesn't match their own.  The same goes for almost any other social insect. Pheromones can also be excreted to attract mates (this applies to animals as well as insects).  Humans are perhaps the only animal that do not poses pheromones) onto the host that acts as "beacon" for the facehugger (this same pheromone trace could be used to indicate to the drone bringing the egg to the newly cocooned host where the host is in the hive).  In such a case the facehugger has no need to "explode" from the egg as it did in ALIEN (where the prospective host was mobile and very much active).  With a cocooned host the facehugger can take its time in climbing onto the host... it doesn't have to catch it's prey since its already caught.

Once dead, the egg once again returns to its opaque form. It should be noted that the Egg is not capable of giving "birth" to the Facehugger immediately following its own birth, since it doesn't have the tentacles yet, so we must assume they grow out over a short period of time, making the Egg capable of sensing intruders.

It was suggested by a cut-scene in ALIEN, that by grabbing a living host (it is unknown whether it is possible with dead hosts), the Alien can, without the help of a queen, transform the host into an egg. This is a process that happens over a small amount of time, perhaps no more than 8-10 hours (take or give a few). This gives the Aliens the ability to create a queen if at least one host is available, this makes the Aliens even more deadly due to the fact that no matter where they are, there is always the possibility that they are going to multiply.