Is it possible to audit admin users? For example: I would like to know if the some admin user has changed any client user registration.
For changes via the interface (and I'm guessing also the remote API, but have not verified), you can see updates under Monitor > Data Log History. It looks like the admin name is not shown in that table, and hence you cannot sort by/search for a specific admin account, but it is shown in the individual records.
Caveat, I was looking at a 3.2 system which does show the admin name; checking a 3.1 system, it does not.