Having recently read "Music Of The Spears", a novel in the Aliens series, I discovered that (and this may not be the only way) the eggs can pick up on human scents.
Somewhere in the book, and it's one line in 310 pages, so no find-ey today, it describes that this sense of smell is so acute, that an egg can pick up a single human scent molecule from around a mile away or so. This struck me as not such an incredibly good idea, since they could then open up when a human was actually a mile away, so it's feasible that they don't rely on just one detection method.
Personally I believe a combination of pheremones/scent, bioelectrical pulses and maybe even sound (in the form of extremely sensitive 'skin' cells) would provide the egg with a great means of proximity detection, each method allowing the egg to detect a human at varying ranges.
"Pain then, white hot, slamming through a ribcage that was nothing more than a fragile lace spiderweb against the teeth that split him from breastbone to gut and twice as deep."