Like many of you, first time taking down my gsg-16 tn. Upon reassembly, remembered a note about that spring from a youtube video I watched. Check to make sure it was there, and it was not. Spent about an hour with HID lights scouring my garage floor and work bench area. Finally decided to call it a night and see how much a replacement would cost. Put the gun back together, springless, put her back in the safe and called it a night.... That's when I happened across this thread....
Tore down a bic pencil for the spring and went to reinstall. With the gun tore down, got my son to hold the light up so I could see what the hell was going on and wouldn't you know it, the slide catch lever spring was captured within the coils of the re-coil spring. Some tiny picks and a few swear words later and she's back in the safe, all parts accounted for.
So two take always. I compared the bic spring to the OEM spring and there is not much difference. I will keep the bic spring on hand for future cleanings. Second take away, if you are in a similar situation to my own, make sure you thoroughly examine all internal components as your spring may just be hiding out as well.