Do you mind linking me to the match history for this game? I can't find any recent games where you played against a Caitlin. Chances are I can download the replay and see if there's anything odd.
I don't think you understand how hard it is to hack the game like you are describing. Since all the game information is stored server side that would mean you'd have to hack Riot's servers. If you had the skill to hack Riot's servers why would you use that access to win bronze games when you could steal credit card numbers or unlock any content in the game that you wanted?
Hacking is extremely, extremely uncommon. It's much, much easier to script since that can be done client side but scripting can't do thinks like manipulate how much you can life steal, it can only give you improved kiting, improved cs, and the ability to dodge/land nearly all skill shots.
The point I'm trying to make is that it is extremely unlikely the person is hacking and much more likely you missed something.
For example, you said the Caitlin had no life steal. I find that very hard to believe because typically the first item bought by all ADCs is doran's blade which gives 3% life steal. It is also very common for players to get the vampirism mastery in the fervor tree which gives 2% life steal. Furthermore, some people even run life steal quints in their runes.
Also, your game knowledge on warlord's bloodless is flawed. It doesn't increase the life steal you already have. It gives between 0.06% and 20% BONUS life steal based on how much health you are missing. For example, if you had 5% life steal from items/masteries and you were near death warlord's bloodless would give you an additional 20% life steal for a total of 25%.
I hope that helps.