dbus-return-values-table is a variable defined in `dbus.el'.
Its value is

#s(hash-table size 65 test equal rehash-size 1.5 rehash-threshold 0.8 data ())

Hash table for temporary storing arguments of reply messages.
A key in this hash table is a list (:serial BUS SERIAL), like in
`dbus-registered-objects-table'. BUS is either a Lisp symbol,
`:system' or `:session', or a string denoting the bus address.
SERIAL is the serial number of the reply message.

The value of an entry is a cons (STATE . RESULT). STATE can be
either `:pending' (we are still waiting for the result),
`:complete' (the result is available) or `:error' (the reply
message was an error message).