The metal casing of many laptops reflects the xrays, hence you can't see anything inside them.

At many airports you can see the x-ray images of the bags and devices as they are scanned/checked.

In my experience there is always a detailed image of the inside of laptops. Color-coded according to material type/density and clearly showing the location of batteries, etc.

I'd imagine the batteries being switched for plastic explosive would be a simple way to take explosives onboard for unsophisticated attackers, turning on rules out that possibility. Not everyone who wants to blow stuff up is a hacker.

Wouldn’t plastic explosive have a different density to real batteries, and thus show up very obviously on the scanner?

The machines are tuned to highlight specific risky materials (such as liquids & gels).

At one point it was common to be asked to turn on laptops etc. But I haven’t seen them do that for years - presumably because the scanners have improved.

LiPo laptop battery is apparently around 1.8-2 g/cm3 (sorry can't find good figures), whilst C4 is 1.73 g/cm3. Not sure what the density resolution is like -- I'd guess it's poor because you can't tell what thickness/type of casing you're looking through from the luggage "xray"; I'd imagine it to be primarily good at relative density.

X-Rays can pass through thick chunks of metal several times the thickness of a laptop. You just need a powerful enough source. I doubt however that such a source can be used safely in an enclosure as open as the securiry check machines.

Not even close to true - just look at the scan monitor next time you fly: you can clearly see inside any electronics going through the scanner