Sounds like a side effect of the Delayed Locking feature. But I think I'd need to know exactly the conditions you are leaving the car in when you leave to go get something. Are you hitting the door power lock button, or the key fob. Is the door open when you do this or closed?
Check your Owner's Manual pages 102-105 for more details.
What made me not so interested in the issue was that you said it behaves like this only "at times". That usually means either something's broken (not so likely if the locks work otherwise) or that you haven't done quite enough experimenting to reproduce the conditions reliably. If you can do it all the time, then describe exactly the chain of events, and it might coincide with what's written in the manual.
Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum
A forum community dedicated to Chevy Trailblazer and GMC Envoy owners and enthusiasts. Come join the discussion about performance, modifications, troubleshooting, towing capacity, maintenance, and more!