dabbrev-check-other-buffers is a variable defined in `dabbrev.el'.
Its value is

Should M-/ look in other buffers?
nil: Don't look in other buffers.
t: Also look for expansions in the buffers pointed out by
Anything else: When we can't find any more expansions in
the current buffer, then ask the user whether to look in other
buffers too.

The default value is t.

You can customize this variable.