Here is a way to obtain a list of disconnected xrdp sessions. It relies on the fact that the xrdp server is, in normal X session manager usage, the only client that establishes a TCP connection to the Xvnc X Window System display server. (The other client programs -- the ones doing things for the user -- use UNIX-domain sockets for their display connections).
When an xrdp session is active, the associated Xvnc display server has two TCP connections, one in the ESTABLISHED state, and the other in the LISTEN state. That looks something like this using the lsof(1) program.
$ sudo lsof -b -w -n -c /^Xvnc$/b -a -iTCP:5900-5999 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME Xvnc 1625 guest 1u IPv4 252910 0t0 TCP 127.0.0.1:5910 (LISTEN) Xvnc 1625 guest 9u IPv4 261226 0t0 TCP 127.0.0.1:5910->127.0.0.1:35242 (ESTABLISHED)
If the user of the remote session abandons it by closing the RDP connection (or, in the case of an Apache Guacamole RDP session