Processes consuming 100% CPU but SPID not in v$process

On my 12.1.0.2 dataguard standby (running on RHEL 7.2), I regularly see high CPU usage, caused by processes which don't show up in v$process.

e.g. right now I'm looking at the output of top, and the top lines are:

8112 oracle    20   0 10.320g  41276  30092 R  99.7  0.3   2:08.38 oracle_8112_****

7329 oracle    20   0 10.332g  44104  26368 R  99.3  0.3  40:56.26 oracle_7329_****

5030 oracle    20   0 10.327g   6380   3644 S   0.7  0.0 295:09.60 ora_pr00_****

Running the query:

select * from v$process where spid in (8112,7329);

Returns no rows.

This is very similar to 285026.1, but I've got EXPIRE_TIME commented out in sqlnet.ora, and this is a _much_ more recent version than 9i!

I've tried running strace on the processes involved, and it just shows a _lot_ of clock requests, presumably for ASH/monitoring purposes.