Most backdoors use obscuring code to hide it, which makes searching for common backdoors a nightmare. Fortunately, if an addon has a backdoor, you'll see it in whatever comment section you found the addon on, unless it lies dormant in wait.
I think you should run each leak on a separate server to see what goes on. I'm sure there some type of log that shows so if it backdoor I'm sure you can find it unless it's different from your typical backdoors.
If you really want to be safe go through all the code and make sure there isn't any sus steam id's or console commands in there. Also you should buy CAC it's a great anti cheat and it has a backdoor checker. I personally buy all my addons because the dev's really deserve the money for it but you do you.