Looking at Google's own Material Guidelines (scroll down to "Control"), we can see that Google themselves do not deem it necessary to include a button. The ways to dismiss the modal are listed as follows:
They can be dismissed by:
- Tapping a menu item or action within the bottom sheet
- Tapping the scrim
- Swiping the sheet down
- Using a close affordance within the bottom sheet’s top app bar, if available
But they mention a close button explicitly for full-screen modules:
Display a close affordance in a full-screen modal bottom sheet.
I have also used a bottom sheet as a menu in my own app for a long while and have never had complaints from users.
Conclusion: I think it is save to assume that such a modal has enough ways to close it. So even if a user expects a button in the first 5 seconds but cannot find it, he will find SOME way to close it, simply because it is so easy.