First question... is there any possibility there is an aftermarket alarm/keyless entry/remote start on it? My aftermarket alarm has the option to do that (granted it's disabled).
As Devin said, I've never seen that feature in a GMT400 pickup from the factory....
GMT400 trucks that I've been in, that don't auto-lock.
1995 K1500 (best friend's brother)
1996 Tahoe (Brandon's old ride)
1997 Sierra K1500 (Devin's cousins, above)
1997 Silverado K2500 (friend)
1998 2-door Tahoe (2nd mom)
1998 Sivlerado 5.7L (best friend)
Heck, even my 02 GMT800 doesn't autolock.