Prowl wasn't told, and did expect Jazz to recognise him when they met in Part 1, which was why he was quite thrown when the mech didn't, and so Optimus did all the talking with Jazz. Also, he's adjusting to being alive again, finding his feet in a place he's not familiar with and being cautious since he knows he doesn't have much by way of resources and so can't exactly storm the Council demanding things be fixed.

Why the Guards were wiped will be coming up eventually (I hope, bunnies being what they are), and as for how, I'm leaving that ambiguous. Much like how the writers of TFA never went into detail into the functioning of the AllSpark. I may not be at their level, but I can still borrow a trick or two ;3