How does F5 LTM route traffic to a subnet it's not aware of

You can disable this feature but it's enabled by default as it can be quite useful and remember it only applies to traffic directed at a Virtual Server. Don't think it's a security risk because it's effectively routing when you don't want it to, it won't do this for other traffic.

 

 

You can globally disable this feature via System> Configuration > Local Traffic > General but obviously, ensure you have relevant TMOS/LTM routes in place first.