McCabe seems to be protecting himself from possible problems with Mueller or needed to gently threaten him.
The problem might be deeper, such as a splintering of interests. Let’s say that the phone was discovered and immediately brought to the attention of the Secret Service. Let’s then propose the fact that the Secret Service may have attempted to determine if this was a threat to the security and privacy of the office of the president and examined the device as evidence before returning it, or perhaps reached out to another agency for information (avoiding the FBI and DOJ) to help ascertain if the president’s safety and privacy were intentionally being compromised. What would happen if these agencies feel that there were bad actors (or at least actions that appear to be bad faith) and the phone episode was considered evidence? Would it even be possible for the Secret Service to run an investigation using other agencies to run counter intelligence FOR the President?