At a little past one last Friday I was passing through the Landmark bridgeway on my way to SM. I noticed that Glorietta's metal detectors were beeping but the security guards weren't doing anything about it. And then I remembered someone telling me that those handheld metal detectors didn't actually work -- they just beeped and were basically there to assuage customers' safety concerns. This I immediately dismissed as urban legend as metal detectors are cheap mechanisms. (you can even build your own)
Anyway, the problem is that they can detect only guns and knives, not bombs. Which makes malls safe from hold-uppers and armed robbers but not bombers. In fact there was a show on RJTV that had their undercover guys smuggling bomb components into various malls proving that the "magic stick" method doesn't really work. A group of people working in concert can bring in and assemble a bomb within a mall.
And that was a household chemical bomb. C-4 (with a blasting cap and a cellphone detonator) may be a little easier to bring in absent bomb-sniffing dogs. Which is what the malls should have if they're concerned about bomb threats. Right now (I was just there this afternoon) they're just poking a little bit deeper.