Detection by airport security might probably be avoided using the right type of explosive. I have no real idea about this, but I suspect that any nation-state with enough budget and know-how can manufacture undetectable or very hard-to-detect explosive devices. If the explosive is encapsulated in a sealed airtight container, which is properly "washed" after manufacturing, I guess there's no way to chemically detect the explosive inside. Not sure about how to avoid X-Ray detection but that's generally not the way explosives are actually detected.

And, is the device anyway going to pass through airport security? I guess the owners are not really travelling on commercial airliners.

These devices apparently were distributed to thousands of operatives. I would imagine that people having those are some of the more elite ones and they probably will travel for business reasons, be it personal business or Hezbollah business. A few who choose to take their pagers with them(i.e. will not be heading straight home after travel, so brings the pager) are huge risk IMHO. Even a single incident may reveal the plot.

I don't know how those detectors at the airports work exactly but they are probably playing cat and mouse game with the people who are into smuggling things and as a result they are probably aware of the more advanced methods like injecting things into the plastic.

This is the reason why I think the explosives were most likely hidden in the batteries. Some explosives have similar enough chemistry that they cannot be told apart from legitimate battery packs by the scanners.

This is a known threat, and this is the reason why some airports do extra checks on some travellers (for example asking them to turn their laptops on, asking them when and where they got the laptop and etc.)

https://en.m.wikipedia.org/wiki/2017_electronics_ban

The scanners only test for the signatures of common chemical structures of explosives, like nitro and nitrate groups, which make up the bulk of mass produced explosives. There are many lesser known chemistries for high explosives that will not be detected by these scanners. Probably the best known example actually used by terrorists are explosives based on peroxide chemistry but there are several others.

A friend of mine who visited Lebanon (and the Hezbollah museum, he gave me a Hezbollah cap by the way) then went to Israel and was subjected to a thorough explosive material search. They basically swiped some kind of broom all over his body (with a focus on genitals) and put it in some device (some kind of spectrometer ?).

This sounds like your regular check for explosives at any other airport in Europe or the US, probably many other places.

news reports suggest 1000+ of these devices exploded.

In a country where lots of people would happily crack open the lid of a device to replace the battery or otherwise tinker, the explosives must have been well hidden, not just tucked into the case.

it is my understanding that the devices had been delivered recently