Comment by svnt
The best evidence we have now suggests that the devices used had removable (AAA) batteries, not built-in batteries.
If I was buying pagers and had previously been hit by intelligence ops I would be buying batteries in random supermarkets.
I'm looking at pager teardowns and there's nothing even close to the volume of the battery in there. Big transistor and the speaker housing.
Which sort of leans back toward the theory that nobody checked the pagers at all.