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While smoke-testing Bustle on a Fedora 32 computer I noticed that dbus-monitor sometimes gets abruptly disconnected from the bus. I found the following in the journal:

May 20 23:26:09 gelf dbus-broker[2628]: Monitor :1.311 is being disconnected as it attempted to send a message.

Sure enough, strace confirms that it's trying to send a message:

$ strace -e write=3 dbus-monitor  --pcap >ohno.pcap
...
sendmsg(3, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="l\2\1\1\0\0\0\0\3\0\0\0\30\0\0\0\6\1s\0\6\0\0\0:1.322\0\0"..., iov_len=40}, {iov_base="", iov_len=0}], msg_iovlen=2, msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 40
 * 40 bytes in buffer 0
 | 00000  6c 02 01 01 00 00 00 00  03 00 00 00 18 00 00 00  l............... |
 | 00010  06 01 73 00 06 00 00 00  3a 31 2e 33 32 32 00 00  ..s.....:1.322.. |
 | 00020  05 01 75 00 12 00 00 00                           ..u.....         |

Deserializing this with GDBusMessage gives:

 Type:    method-return
 Flags:   no-reply-expected
 Version: 0
 Serial:  3
 Headers:
   reply-serial -> uint32 18
   destination -> ':1.322'
 Body: ()
 UNIX File Descriptors:
   (none)

Rather suspiciously, the last few messages in the pcap file are:

1. AddMatch from :1.322 to org.freedesktop.DBus, serial 17
2. org.freedesktop.DBus responding to that method call
3. org.freedesktop.DBus responding to the invisible call with serial 18
4. org.freedesktop.DBus.Local.Disconnected

That's as far as I got. :(