The.weight has been the subject of.previous debate.
Traditionally, generally, there is has been no real time weighing of aircraft going on during loading...the overall weight needs to be known for.performance reasons but is usually derived by summing the measured weight of fuel with the known or assumed weights, as applicable, for things like the hull, cargo, passengers etc...that normally works well enough, but it's true that if passengers numbers are wrong the calculated overall weight is out and the crew may never know.
A previous poster has mentioned some aircraft can weigh themselves....now I know that has been trialled over the years but where that's got to, whether it's now widely in use or used by Jet2, don't know.
If you did have instantaneous measured weight available the underload should have been obvious, but to remphasise there's no info I've seen that it was available here, and if it was not available there's no way for the flight crew to spot the underload.
ETA: an.unknown underload of several tonnes isn"t great but not the end of the world...the bigger problem would be if the missing passengers shifted the aircraft's centre of gravity (balance point) significantly from where it was supposed to be.