No, there is not an external antenna for the NodeMCU WiFi module. The antenna is built-in to the wifi module. It's the squiggly copper line above the WiFi chip.
Do Not Modify the Antenna
Modifying the antenna invalidates the FCC certification of the module and we cannot encourage it or support it.
If your metal enclosure is adversely affecting the WiFi signal to your device, we suggest either leaving the door of the enclosure off, or replacing the enclosure with a plastic one. You can also mount the Konnected devices directly to a wall or wooden board.
Plastic Enclosures on Amazon
  | uuxcell 10.04" x 7.87" x 2.99"(255mmx200mmx76mm) ABS Junction Box Universal Project Enclosure White |
  | uxcell 7.48"x7.4"x2.76"(190mmx188mmx70mm) ABS Junction Box Universal Project Enclosure |