In the legacy version of Sneak Thief (0.99) there is a glitch where the guards basically turn deaf, alarms don't trigger them, throwing grenades don't trigger them, and you shooting a non-silenced gun doesn't trigger them either, however if they directly see you, they will still chase you. Also, if a guard sees you, then it'll alert all nearby guards straight to you for whatever reason. Oh, and also walking through lasers also triggers them (unless it's one that's hooked up to a turret instead of an alarm, like the one on the 4th floor of The Data Center, in which case your gonna die anyways).

I first discovered this glitch by accident, I was practicing doing an Any% run, when in "The Stash House" level, I realized that when I kept shooting, the guards down in that locked door wouldn't barge through the door like they normally do, originally I thought this was just the game being broken, but then when I went back in the level after reloading the game, then they would actually hear me, and barge through that door, I have tried to figure out what caused the glitch, but I think I know now.

Literally all you need to do, is just enter The Mansion level, yeah that's it, that's all, if you exit or complete this level, then you can go into any stage you want (with the exception of all the stages with police cars, which in this case is The Jewelry Store, and The Neighborhood), and the guards will be deaf, the only way to remove this glitch being to exit the entire game, and starting it back up. This is useful for some levels, and not others, The Stash House has lots of time save from this glitch, but using it on any level where you plan on making noise, and luring the guards to get cheap kills, as some kind of safety strategy, you don't want this glitch.