According to the Everspring SM103 Sensor manual:
7. If several windows need to be protected, adopt the wire according to the
specifications as mentioned below. This should be wired to the terminal block
provided in the battery compartment in series connection. The wired contact
should be connected using two core (24AWG) wire of maximum length 4m.
A cable entry cut-out is available and adjacent to the terminal block.
Internal connection External wired connection
Choose either the internal or external wired connection, both of which types cannot
be in existence at the same time.
Did anyone already use the SM103 with an "external wired connection"?
My guess is that you could use a reed contact as external connection.
I found two contacts at Conrad, the RS-168-TS an NC version and the RS-128B an NO version of a reed contact.
Who knows what version is the right one?