You are able to kill up to 3 guards without any ill effects. Once you kill the fourth guard, one additional guard will show up. This functions the same way as it does on Framing Frame day 1. I don't know for certain if there are many locations the guard will spawn from, but he does appear from the very bottom of the staircase (the part that isn't accessible).
While stealthed, Bain will announce the arrival of the extra guard. Once he appears, he will inspect the entire place (he follows a similar path to other guards). He is noticeable because he stops to talk into his walkie-talkie and report that things are clear. Once he has inspected the entire place (takes a few minutes), he will leave. Again, this function the same way on day 1.
To the best of my knowledge, this functions the same on all difficulties (although I believe there are less guards to begin with on lower difficulties), but truthfully I've never completed the map on Normal and it has been a long time since I've done anything less than Overkill. Speaking of to the best of my knowledge, this information all comes from hours of playing; however, I haven't played the game in about 6 months, so it may be patched (Overkill don't explicitly mention every change they make in their patch notes unfortunately).
Since this functionality wasn't in the game to begin with, I went through the patch notes but couldn't find much to support any of this unfortunately. Here's all I got:
- During stealth Bain now informs the crew if a new guard is about to arrive
- Changed the timer on the spawning patrol guards to make it more balanced for each difficulty