There aren't any such entries in syslog, presumably because I had hardcoded two upstream servers (208.67.222.222 and 208.67.220.220) using the GUI Wi-Fi settings dialog in 16.10 and they're not changing. Oddly, I can't see that setting in the 17.04 dialog, even though "systemd-resolve --status" correctly reports them:

Global
         DNS Servers: 127.0.0.1
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 2 (wlp2s0)
      Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: allow-downgrade
    DNSSEC supported: no
         DNS Servers: 208.67.222.222
                      208.67.220.220
          DNS Domain: local

The requests against the upstream server are disturbingly fast, I'm surprised I haven't been blacklisted yet.

19:40:12.000415 IP hostname.50776 > resolver1.opendns.com.domain: 42051+% [1au] PTR? 4.1.168.192.in-addr.arpa. (53)
E..Q..@.@........C...X.5.=nB.C...........4.1.168.192.in-addr.arpa.......)........
19:40:12.000920 IP hostname.59219 > resolver1.opendns.com.domain: 14223+% [1au] PTR? 4.1.168.192.in-addr.arpa. (53)
E..Q..@.@........C...S.5.=..7............4.1.168.192.in-addr.arpa.......)........
19:40:12.001411 IP hostname.51647 > resolver1.opendns.com.domain: 2501+% [1au] PTR? 4.1.168.192.in-addr.arpa. (53)
E..Q..@.@........C.....5.=.Z	............4.1.168.192.in-addr.arpa.......)........
19:40:12.001885 IP hostname.33104 > resolver1.opendns.com.domain: 30929+% [1au] PTR? 4.1.168.192.in-addr.arpa. (53)
E..Q..@.@........C...P.5.=..x............4.1.168.192.in-addr.arpa.......)........
19:40:12.002412 IP hostname.47231 > resolver1.opendns.com.domain: 46563+% [1au] PTR? 4.1.168.192.in-addr.arpa. (53)
E..Q..@.@........C.....5.=j{.............4.1.168.192.in-addr.arpa.......)........
19:40:12.004238 IP hostname.57292 > resolver1.opendns.com.domain: 61082+% [1au] PTR? 4.1.168.192.in-addr.arpa. (53)
E..Q..@.@........C.....5.=
w.............4.1.168.192.in-addr.arpa.......)........
19:40:12.008187 IP hostname.49786 > resolver1.opendns.com.domain: 14681+% [1au] PTR? 4.1.168.192.in-addr.arpa. (53)
E..Q..@.@........C...z.5.=.
9Y...........4.1.168.192.in-addr.arpa.......)........
19:40:12.008926 IP hostname.53171 > resolver1.opendns.com.domain: 24423+% [1au] PTR? 4.1.168.192.in-addr.arpa. (53)
E..Q..@.@........C.....5.=.._g...........4.1.168.192.in-addr.arpa.......)........
19:40:12.009629 IP hostname.37811 > resolver1.opendns.com.domain: 55231+% [1au] PTR? 4.1.168.192.in-addr.arpa. (53)
E..Q..@.@........C.....5.=mk.............4.1.168.192.in-addr.arpa.......)........