Steam Overlay does not show in Win64 Shipping builds when run outside of Steam. The Steam client can be running, but unless the .exe is opened through Steam itself, the Overlay will not appear. Note that when opened by adding the .exe to the Steam library as noted in the documentation below, the overlay appears as expected.
According to the documentation here, this is expected, but only on the Mac:
Reproduced in 4.7.6 binary, 4.8.3 binary, 4.10.0 binary, and Main (CL# 2761843)
1. Open the Multiplayer Shootout project
2. Package for Shipping Win64
3. Open the Steam Client
4. Open the packaged project's .exe
The Steam Overlay does not appear
Head over to the existing AnswerHub thread and let us know what's up.