Free chlorine loss

Just curious what could be depleting my free chlorine levels?

We got in the hot tub on Wednesday and FC was 4.5 after getting our. So I did not add anymore chlorine. Hot tub stayed covered until today when we got in and FC was 0. CC was also 0. So I’m left wondering how it got to 0.

Could it be the case that FC was still being used after we got out and I tested to clean contaminants we left from being in there Wednesday night?