Judging by this thread, I’d say the SM is the cause of my memory leak: MacOS Sierra 10.12.4 Memory leak - #3 by MikeWayzovski.
I did rather fear that investing in a SM might raise quite a few teething troubles. As things stand, I have a SM that does nothing but move the cursor around in a rather less manageable way than my regular mouse and a memory leak problem. Oh the joys of tech!