The security light is an indicator for a failed antitheft system. This is ALL it is for. NOTHING else. If the light is on and you are experiencing no adverse effects, you are probably alright. My guess is that the system failed, and somebody bypassed the antitheft system. (poorly) This would cause the light to be constant and the vehicle to run normally. (assuming you bought this used)
Here is a cheap way to see if the system was bypassed.
Go get a copy made of your truck key. Will cost a couple bucks. The original key has a Transponder hidden in the plastic head of the key. This transponder is a needed part of the antitheft system. The new key will NOT have this transponder. If it turns fine and starts the truck like normal, then your truck is bypassed and problem is explained and solved. If the new key turns but the truck will not remain running, then you are probably on the verge of a security failure, indicated by the security light.
Also make sure the original key is nowhere near the vehicle when you try this out. You dont want the ignition reading the signal from the original key while trialing the newly copied key
This link explains the SECURITY issue more.http://vatspasslockpasskeysecurityhelp.yolasite.com/