I've never seen it report a clock that wasn't being used as one. If I don't know what it's driving, I do a quick:
create_clock -period 1.0 -name test [get_keepers {}]
(It may be get_ports, or something like that). Then just report_timing to that new clock.
The report is there to help you. There is nothing in .sdc to say, I'm not a clock, you just choose not to constrain it. The Unconstrained Paths report is outside of .sdc constraints, and really just trying to help you identify possible mistakes. I would ignore it if it is a clock and you just don't want to constrain it.
(A final option is to put a clock constraint on it that's 1000.0 ns, then do:
set_false_path -to [get_clocks test_clk]
set_false_path -from [get_clocks test_clk]
It should get rid of the info message since it's constrained, and it wont' be analyzed.